Semi-sync status were not reset by FLUSH STATUS, this was because
all semi-sync status variables are defined as SHOW_FUNC and FLUSH
STATUS could only reset SHOW_LONG type variables.

This problem is fixed by change all status variables that should
be reset by FLUSH STATUS from SHOW_FUNC to SHOW_LONG.

After the fix, the following status variables will be reset by
FLUSH STATUS:
  Rpl_semi_sync_master_yes_tx
  Rpl_semi_sync_master_no_tx

Note: normally, FLUSH STATUS itself will be written into binlog
and be replicated, so after FLUSH STATS, one of
  Rpl_semi_sync_master_yes_tx
  Rpl_semi_sync_master_no_tx
can be 1 dependent on the semi-sync status. So it's recommended
to use FLUSH NO_WRITE_TO_BINLOG STATUS to avoid this.