Ensure that user sees all the data before connection is closed

If user throttles receive by setting recv_mbuf_limit,
after the net interface reports connection as closed we must wait
for data to trickle through before disposing of it.
There can still b data in the buffers (e.g. SSL).

CL: mg: Ensure that user sees all the data before connection is closed

PUBLISHED_FROM=22be0fa368950a9fdb03cfb00febc7c0a1674b01

docs/c-api/mg_net.h/struct_mg_connection.md

@@ -46,6 +46,7 @@ signature: |
   #define MG_F_WANT_READ (1 << 6)          /* SSL specific */
   #define MG_F_WANT_WRITE (1 << 7)         /* SSL specific */
   #define MG_F_IS_WEBSOCKET (1 << 8)       /* Websocket specific */
+  #define MG_F_RECV_AND_CLOSE (1 << 9) /* Drain rx and close the connection. */
   
   /* Flags that are settable by user */
   #define MG_F_SEND_AND_CLOSE (1 << 10)      /* Push remaining data and close  */