MEDIUM: compression: use pool for comp_ctx

Use pool for comp_ctx, it is allocated during the comp_algo->init().
The allocation of comp_ctx is accounted for in the zlib_memory_available.
diff --git a/include/types/session.h b/include/types/session.h
index 284b7e8..e0d3c22 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -159,7 +159,7 @@
 	void (*srv_error)(struct session *s,	/* the function to call upon unrecoverable server errors (or NULL) */
 			  struct stream_interface *si);
 	unsigned int uniq_id;			/* unique ID used for the traces */
-	struct comp_ctx comp_ctx;		/* HTTP compression context */
+	struct comp_ctx *comp_ctx;		/* HTTP compression context */
 	struct comp_algo *comp_algo;		/* HTTP compression algorithm if not NULL */
 	char *unique_id;			/* custom unique ID */
 };