The function wsrep_on() was being called rather frequently
in InnoDB and XtraDB. Let us cache it in trx_t and invoke
trx_t::is_wsrep() instead.

innobase_trx_init(): Cache trx->wsrep = wsrep_on(thd).

ha_innobase::write_row(): Replace many repeated calls to current_thd,
and test the cheapest condition first.