replication: disconnect applier on timeout

In replication schema if one of the instances was powered off, it isn't
detected by others and the connection hangs. Alive machines show
'follow' state. Add timeout to solve this issue. It's safe since
applier and relay both send messages every replication_timeout so we can
assume that if we read nothing we have problem with connection. Use
replication_disconnect_timeout which is replication_timeout * 4 as for
now.

The test fixed and comments improved by @locker.

Closes #3025