Fix COPY hangs.

During COPY IN state (i.e. COPY FROM STDIN), frontend can send Flush
or Sync messages.  According to the F/B protocol specification, they
should be ignored but Pgpool-II treated as an invalid message and this
causes COPY hung.

Discussion: #79
Backpatch-through: v4.1

src/protocol/pool_proto_modules.c

@@ -3398,6 +3398,12 @@ CopyDataRows(POOL_CONNECTION * frontend,
 					copy_count++;
 					continue;
 				}
+				/*
+				 * Flush (H) or Sync (S) messages should be ignored while in
+				 * the COPY IN mode.
+				 */
+				else if (kind == 'H' || kind == 'S')
+					continue;
 				else
 				{
 					if (pool_config->log_client_messages && copy_count != 0)