Fix for CONC-659:

When checking for a semi sync indication header, we need also check if
the undocumented session variable @rpl_semi_sync_slave was set.
Otherwise the timestamp of the event could contain values which match
the 2 bytes of the semi sync header.

Since the variable rpl_semi_sync_slave and it's behavior is not documented,
a new option MARIADB_RPL_SEMI_SYNC was added.

libmariadb/ma_errmsg.c

@@ -117,6 +117,7 @@ const char *mariadb_client_errors[] =
   /* 5020 */ "Binary log error (File: %.*s start_pos=%ld): Unknown event type (%d) with flag 'not_ignorable'.",
   /* 5021 */ "Binary log error (File: %.*s start_pos=%ld): %s.",
   /* 5022 */ "File '%s' is not a binary log file",
+  /* 5023 */ "Semi sync request error: %s",
   ""
 };