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/proto/compression.h b/include/proto/compression.h
index cfab62a..1614edd 100644
--- a/include/proto/compression.h
+++ b/include/proto/compression.h
@@ -33,22 +33,21 @@
 int http_compression_buffer_add_data(struct session *s, struct buffer *in, struct buffer *out);
 int http_compression_buffer_end(struct session *s, struct buffer **in, struct buffer **out, int end);
 
-int identity_init(struct comp_ctx *comp_ctx, int level);
+int identity_init(struct comp_ctx **comp_ctx, int level);
 int identity_add_data(struct comp_ctx *comp_ctx, const char *in_data, int in_len, struct buffer *out);
 int identity_flush(struct comp_ctx *comp_ctx, struct buffer *out, int flag);
 int identity_reset(struct comp_ctx *comp_ctx);
-int identity_end(struct comp_ctx *comp_ctx);
+int identity_end(struct comp_ctx **comp_ctx);
 
 
 #ifdef USE_ZLIB
-
-int deflate_init(struct comp_ctx *comp_ctx, int level);
+int deflate_init(struct comp_ctx **comp_ctx, int level);
 int deflate_add_data(struct comp_ctx *comp_ctx, const char *in_data, int in_len, struct buffer *out);
 int deflate_flush(struct comp_ctx *comp_ctx, struct buffer *out, int flag);
 int deflate_reset(struct comp_ctx *comp_ctx);
-int deflate_end(struct comp_ctx *comp_ctx);
+int deflate_end(struct comp_ctx **comp_ctx);
 
-int gzip_init(struct comp_ctx *comp_ctx, int level);
+int gzip_init(struct comp_ctx **comp_ctx, int level);
 #endif /* USE_ZLIB */
 
 #endif /* _PROTO_COMP_H */