Reduce number of wsrep calls to server code from InnoDB

Thread executing wsrep transaction can't change during transaction execution. Similarly, thread executing high priority brute force (BF) transaction does not change during transaction execution. Therefore, in both cases there is no need to call server code after transaction has initialized.

InnoDB already stores information is this wsrep transaction to trx_t::wsrep and this is checked using trx->is_wsrep() function. Because, brute force transaction is always a wsrep transaction we can extend trx_t::wsrep variable so that value

    0 == not wsrep transaction (and not BF)

    1 == normal wsrep transaction

    2 == high priority BF wsrep transaction

    4 == high priority BF transaction is performing unique secondary index scan