applier: check sync directly (not from writer)

We check if it's time to switch the applier to the FOLLOW state from
applier writer, because we might need to call on_state triggers, which
may yield. The fact is FOLLOW triggers never yield so this isn't
necessary. Let's move applier_check_sync calls out of applier writer.

This is a step towards moving the applier fiber from the main to the
applier thread, which is required to properly handle SSL connections.

NO_DOC=refactoring
NO_CHANGELOG=refactoring