applier: extract plain tx application from applier_apply_tx()

The new routine, called apply_plain_tx(), may be used not only by
applier_apply_tx(), but also by final join, once we make it
transactional, and recovery, once it's also turned transactional.

Also, while we're at it. Remove excess fiber_gc() call from
applier_subscribe loop. Let's better make sure fiber_gc() is called on
any return from applier_apply_tx().

Prerequisite #5874
Part of #5566