lbox_push_event_f and lbox_push_event_f callback functions used for
passing the statement between txn and space on/before replace Lua
triggers don't assume that the transaction may be aborted by yield
after the current statement began (this may happen if a trigger callback
yields). In this case, all statements in txn would be rolled back and
txn_current_stmt would return NULL, leading to a crash.

Let's fix this by checking if the transaction is still active and
raising an error immediately if it isn't, thus skipping Lua triggers.

Notes:
 - We merged lbox_pop_txn_stmt_and_check_format into lbox_pop_txn_stmt,
   because the latter is only called by the former.
 - Since lbox_push_event_f callback may now fail, we have to update
   lbox_trigger_run to handle it.

Closes #8027

NO_DOC=bug fix