DOC: lua: schematics about lua socket organization

This schema shows the structs and the connections between these structs
for implementing the socket systems.
diff --git a/doc/internals/lua_socket.fig b/doc/internals/lua_socket.fig
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+++ b/doc/internals/lua_socket.fig
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+#FIG 3.2  Produced by xfig version 3.2.5b
+Landscape
+Center
+Metric
+A4      
+100.00
+Single
+-2
+1200 2
+6 7020 8775 9675 9405
+4 0 0 50 -1 12 12 0.0000 4 165 2400 7020 8910 update_tcp_handler()\001
+4 0 0 50 -1 16 12 0.0000 4 195 2640 7020 9105 Called on each change on the \001
+4 0 0 50 -1 16 12 0.0000 4 195 1830 7020 9345 tcp connection state.\001
+-6
+6 7020 9675 10170 10080
+4 0 0 50 -1 12 12 0.0000 4 165 2160 7020 9810 hlua_tcp_release()\001
+4 0 0 50 -1 16 12 0.0000 4 195 3150 7020 10005 Called when the applet is destroyed.\001
+-6
+6 765 8730 3195 9450
+4 0 0 50 -1 12 12 0.0000 4 165 1560 765 8910 hlua_tcp_gc()\001
+4 0 0 50 -1 16 12 0.0000 4 195 2430 765 9105 Called just before the object\001
+4 0 0 50 -1 16 12 0.0000 4 195 840 765 9345 garbaging\001
+-6
+6 900 3555 2340 4365
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 900 3555 2340 3555 2340 4365 900 4365 900 3555
+4 0 0 50 -1 16 12 0.0000 4 180 1080 990 4005 lua_State *T\001
+4 0 0 50 -1 18 12 0.0000 4 150 990 990 3735 struct hlua\001
+4 0 0 50 -1 16 12 0.0000 4 195 1245 990 4275 stop_list *stop\001
+-6
+2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 1 0 2
+	1 1 1.00 60.00 120.00
+	 10530 6750 8910 6570
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 10440 6390 13320 6390 13320 6930 10440 6930 10440 6390
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+	 6480 2745 6480 10035
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+	1 1 1.00 60.00 120.00
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+	 6885 7110 6840 7155 6840 7335 6885 7380
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+	1 1 1.00 60.00 120.00
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+	1 1 1.00 60.00 120.00
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+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 0 5040 2520 5040 2520 7830 0 7830 0 5040
+2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 2
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+	 90 6120 2430 6120 2430 6975 90 6975 90 6120
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
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+	 6705 6480 8910 6480 8910 8010 6705 8010 6705 6480
+2 1 0 1 0 7 50 -1 -1 0.000 0 0 -1 1 0 2
+	1 1 1.00 60.00 120.00
+	 6840 7605 2430 6840
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+	 810 3015 2430 3015 2430 4455 810 4455 810 3015
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 6795 6750 8820 6750 8820 7920 6795 7920 6795 6750
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 3060 630 4500 630 4500 1440 3060 1440 3060 630
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 2970 90 4635 90 4635 1575 2970 1575 2970 90
+2 2 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 5
+	 10350 6120 13410 6120 13410 7020 10350 7020 10350 6120
+3 0 1 1 13 7 50 -1 -1 1.000 0 1 0 2
+	5 1 1.00 60.00 120.00
+	 6885 8010 6885 8910
+	 0.000 0.000
+3 0 1 1 13 7 50 -1 -1 1.000 0 1 0 3
+	5 1 1.00 60.00 120.00
+	 6750 8010 6750 9675 6885 9810
+	 0.000 1.000 0.000
+3 0 0 1 0 7 50 -1 -1 0.000 0 1 0 5
+	1 1 1.00 60.00 120.00
+	 990 3915 540 4140 495 4365 540 4725 585 5040
+	 0.000 1.000 1.000 1.000 0.000
+3 0 1 1 13 7 50 -1 -1 1.000 0 1 0 3
+	5 1 1.00 60.00 120.00
+	 450 7830 450 8595 675 8914
+	 0.000 1.000 0.000
+4 0 0 50 -1 18 12 0.0000 4 195 2565 10530 6570 struct stream_interface si[0]\001
+4 0 0 50 -1 16 12 0.0000 4 195 1725 10530 6840 enum obj_type *end\001
+4 0 0 50 -1 18 12 0.0000 4 150 885 90 5220 stack Lua\001
+4 0 0 50 -1 16 12 0.0000 4 195 1140 90 5490 stack entry 0\001
+4 0 0 50 -1 16 12 0.0000 4 195 1140 90 5760 stack entry 1\001
+4 0 0 50 -1 16 12 0.0000 4 195 1140 90 6030 stack entry 2\001
+4 0 0 50 -1 18 12 0.0000 4 195 1200 6795 6660 struct appctx\001
+4 0 0 50 -1 18 12 0.0000 4 195 1695 180 6300 struct hlua_socket\001
+4 0 0 50 -1 16 12 0.0000 4 150 1470 180 6570 struct session *s\001
+4 0 0 50 -1 16 12 0.0000 4 195 1140 90 7380 stack entry 3\001
+4 0 0 50 -1 16 12 0.0000 4 195 1140 90 7740 stack entry 4\001
+4 1 12 50 -1 12 9 5.6723 4 135 540 2925 4545 (list)\001
+4 0 0 50 -1 18 12 0.0000 4 195 2205 3465 4995 struct hlua_socket_com\001
+4 1 12 50 -1 12 9 5.5851 4 135 540 5265 5760 (list)\001
+4 0 0 50 -1 18 12 0.0000 4 150 1305 900 3240 struct session\001
+4 0 0 50 -1 16 12 0.0000 4 150 1440 900 3465 struct task *task\001
+4 0 0 50 -1 16 12 0.0000 4 150 1440 3465 5220 struct task *task\001
+4 0 0 50 -1 18 12 0.0000 4 150 1110 6885 6930 struct <lua>\001
+4 0 0 50 -1 16 12 0.0000 4 195 1620 6885 7425 struct hlua_tcp *wr\001
+4 0 0 50 -1 16 12 0.0000 4 195 1590 6885 7200 struct hlua_tcp *rd\001
+4 0 0 50 -1 16 12 0.0000 4 180 1845 6885 7650 struct hlua_socket *s\001
+4 0 0 50 -1 18 12 0.0000 4 195 1470 3060 270 struct hlua_task\001
+4 0 0 50 -1 16 12 0.0000 4 150 1440 3060 540 struct task *task\001
+4 0 0 50 -1 16 12 0.0000 4 180 1080 3150 1080 lua_State *T\001
+4 0 0 50 -1 18 12 0.0000 4 150 990 3150 810 struct hlua\001
+4 0 0 50 -1 16 12 0.0000 4 195 1245 3150 1350 stop_list *stop\001
+4 0 0 50 -1 18 12 0.0000 4 150 1305 10440 6300 struct session\001