MEDIUM: use pool for zlib

Don't use the zlib allocator anymore, 5 pools are used for the zlib
compression. Their sizes depends of the window size and the memLevel in
deflateInit2.
diff --git a/include/types/compression.h b/include/types/compression.h
index 097c42c..fa251f1 100644
--- a/include/types/compression.h
+++ b/include/types/compression.h
@@ -38,6 +38,11 @@
 struct comp_ctx {
 #ifdef USE_ZLIB
 	z_stream strm; /* zlib stream */
+	void *zlib_deflate_state;
+	void *zlib_window;
+	void *zlib_prev;
+	void *zlib_pending_buf;
+	void *zlib_head;
 #endif /* USE_ZLIB */
 };