raft: make followers notice leader hang

It's possible to hang an instance by some non-yielding request. The
simplest example is `while true do end`. A more true to life one would
be a `select{}` from a large space, or `pairs` iteration over a space
without yields.

Any such request makes the instance unresponsive - it can serve neither
reads nor writes. At the same time, the instance appears alive to other
cluster members: relay thread used to communicate with others is not
hung and continues to send heartbeats every replication_timeout.

The problem is the most severe with Raft leader elections: followers
believe the leader is fine and do not start elections despite leader
being unable to serve reads or writes.

Closes #7512

NO_DOC=bugfix