lib-imap: When sending "+ OK" to client, uncork ostream immediately

This makes sure that it's sent to the client as soon as possible.

Also removed o_stream_nflush(), since uncorking does that already.
If there is no corking, the "+ OK" was already attempted to be sent
by the o_stream_nsend() call and a flush is unlikely to help.

src/lib-imap/imap-parser.c

@@ -414,7 +414,12 @@ static bool imap_parser_literal_end(struct imap_parser *parser)
 
 		if (parser->output != NULL && !parser->literal_nonsync) {
 			o_stream_nsend(parser->output, "+ OK\r\n", 6);
-			o_stream_nflush(parser->output);
+			if (o_stream_is_corked(parser->output)) {
+				/* make sure this continuation is sent to the
+				   client as soon as possible */
+				o_stream_uncork(parser->output);
+				o_stream_cork(parser->output);
+			}
 		}
 	}