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/compression.h b/include/types/compression.h
index cf56e45..e4b1f27 100644
--- a/include/types/compression.h
+++ b/include/types/compression.h
@@ -50,11 +50,11 @@
 struct comp_algo {
 	char *name;
 	int name_len;
-	int (*init)(struct comp_ctx *comp_ctx, int level);
+	int (*init)(struct comp_ctx **comp_ctx, int level);
 	int (*add_data)(struct comp_ctx *comp_ctx, const char *in_data, int in_len, struct buffer *out);
 	int (*flush)(struct comp_ctx *comp_ctx, struct buffer *out, int flag);
 	int (*reset)(struct comp_ctx *comp_ctx);
-	int (*end)(struct comp_ctx *comp_ctx);
+	int (*end)(struct comp_ctx **comp_ctx);
 	struct comp_algo *next;
 };