REORG: include: move common/memory.h to haproxy/pool.h

Now the file is ready to be stored into its final destination. A few
minor reorderings were performed to keep the file properly organized,
making the various sections more visible (cache & lockless).

In addition and to stay consistent, memory.c was renamed to pool.c.
diff --git a/Makefile b/Makefile
index f7155d0..6e4cad6 100644
--- a/Makefile
+++ b/Makefile
@@ -802,7 +802,7 @@
        src/payload.o src/vars.o src/debug.o src/mux_pt.o src/http_act.o       \
        src/h2.o src/queue.o src/fd.o src/proto_uxst.o src/lb_chash.o          \
        src/ring.o src/frontend.o src/raw_sock.o src/xprt_handshake.o          \
-       src/htx.o src/memory.o src/applet.o src/channel.o src/signal.o         \
+       src/htx.o src/pool.o src/applet.o src/channel.o src/signal.o           \
        src/lb_fwrr.o src/ev_select.o src/sink.o src/http_conv.o               \
        src/proto_sockpair.o src/mworker-prog.o src/activity.o src/lb_fwlc.o   \
        src/http.o src/lb_fas.o src/uri_auth.o src/thread.o src/regex.o        \