raft: move worker fiber from raft library to box

Worker fiber is used by raft library to perform yielding tasks
like WAL write, and simply long tasks like network broadcast. That
allows not to block the raft state machine, and to collect
multiple updates during an event loop iteration to flush them all
at once.

While the worker fiber was inside raft library, it wasn't possible
to use it for anything else. And that is exactly what is going to
be needed. The reason chain is quite long.

It all starts from that the elimination of all box appearances
from raft library also includes relocation of
box_update_ro_summary().

The only place it can be moved to is box_raft_on_update trigger.

The trigger is currently called from the raft worker fiber. It
means, that between raft state update and trigger invocation there
is a yield. If box_update_ro_summary() would be blindly moved to
the trigger, users sometimes could observe miracles like instance
role being 'follower', but the node is still writable if it was a
leader before, because box_raft_on_update wasn't invoked yet, and
it didn't update RO summary.

Assume, the on_update triggers are invoked by raft not in the
worker fiber, but right from the state machine. Then
box_update_ro_summary() would always follow a state change without
a yield.

However that creates another problem - the trigger also calls
box_clear_synchro_queue(), which yields. But on_update triggers
must not yield so as not to block the state machine.

This can be easily solved if it would be possible to schedule
box_clear_synchro_queue() from on_update trigger to be executed
later.

And after this patch it becomes possible, because the worker fiber
now can be used not only to handle raft library async work, but
also for box-raft async work, like the synchro queue clearance.

Part of #5303