MINOR: spoe: Count the number of frames waiting for an ack for each applet

So it is easier to respect the max_fpa value. This is no more the maximum frames
processed by an applet at each loop but the maximum frames waiting for an ack
for a specific applet.

The function spoe_handle_processing_appctx has been rewritten accordingly.
diff --git a/include/types/spoe.h b/include/types/spoe.h
index 2354f6e..659dd27 100644
--- a/include/types/spoe.h
+++ b/include/types/spoe.h
@@ -336,6 +336,7 @@
 	struct buffer_wait  buffer_wait;    /* position in the list of ressources waiting for a buffer */
 	struct list         waiting_queue;  /* list of streams waiting for a ACK frame, in sync and pipelining mode */
 	struct list         list;           /* next spoe appctx for the same agent */
+	unsigned int        cur_fpa;
 
 	struct {
 		struct spoe_context *ctx;    /* SPOE context owning the fragmented frame */