applier: use fiber_cond instead of fiber_channel

Apart from being an overkill, using a fiber_channel object for notifying
about applier state changes is actually unsafe, because fiber_channel
methods involve memory allocations and hence may fail due to OOM while
not methods using them expect this. For instance, applier_disconnect()
should never fail, but it actually may as it calls fiber_channel_put()
via applier_set_state().

To avoid unpredictable behavior caused by unhandled exceptions, let's
switch to plain and simple fiber_cond.