replication: applier state channel can be closed

If applier state channel is closed by connect_all cleanup code then
applier_connect function could throw an unwanted exception on a write
to this channel.

Interrupt applier_pause() on fiber_cancel().

Applier orchestration uses fiber_cancel() to stop applier,
so applier_pause() should exit if the fiber is cancelled.

Add test.

Fixes #2991.