Use isc_buffer_t to keep track of incoming POST data

This commit replaces the ad-hoc 64K buffer for incoming POST data with
isc_buffer_t backed by dynamically allocated buffer sized accordingly
to the value in the "Content-Length" header.

lib/isc/netmgr/netmgr-int.h

@@ -803,6 +803,7 @@ typedef struct isc_nmsocket_h2 {
 	uint8_t *buf;
 	size_t bufsize;
 	size_t bufpos;
+	isc_buffer_t rbuf;
 
 	int32_t stream_id;
 	isc_nm_http_session_t *session;