limbo: rename txn_limbo_apply

The new name is txn_limbo_req_commit(). The reason is that the
future commits will introduce txn_limbo_req_prepare() and
txn_limbo_req_rollback().

The reason is that at least PROMOTE and DEMOTE requests need to
signal their start somehow before going into WAL so as new
synchronous transactions couldn't interfere. It is already done
in txn_limbo_write_promote() and txn_limbo_write_demote(). But not
done when a synchro command comes from an applier.

The new pipeline will be that both local and remote synchro
commands will make a prepare() before WAL write, and then either
commit() or rollback() of the request + commit/rollback() of the
limbo state (txn_limbo_begin/commit/rollback).

Essentially, txn_limbo_req_*() will be similar to txn_stmt_*(),
and txn_limbo_begin/commit/rollback() will be similar to
txn_begin/commit/rollback().

That is also going to be needed to remove panic() from limbo WAL
write failures.

Part of #6842

NO_DOC=Refactoring
NO_TEST=Refactoring
NO_CHANGELOG=Refactoring