raft: add strict fencing

With current leader fencing implementation old leader doesn't resign
it's leadership before new leader may be elected. Because of this
several "leaders" might coexist in replicaset for some time.

This commit changes replication_disconnect_timeout that it is twice
as short for current raft leader (2*replication_timeout) if strict
fencing is enabled. Assuming that replication_timeout is the same for
every replica in replicaset this makes it less probable that new
leader can be elected before old one resigns it's leadership.

Old fencing behaviour can be enabled by setting fencing to soft mode.
This is useful when connection death timeouts shouldn't be affected
(e.g. different replication_timeouts are set to prioritize some
replicas as leader over the others).

Closes #7110

@TarantoolBot document
Title: Strict fencing

In `box.cfg` option `election_fencing_enabled` is deprecated in favor
of `election_fencing_mode`. `election_fencing_mode` can be set to one
of the following values:
'off' - fencing turned off (same as `election_fencing_enabled` set to
false before).
Connection death timeout is 4*replication_timeout for all nodes.

'soft' (default) - fencing turned on, but connection death timeout is
the same for leader and followers in replicaset. This is enough to
solve cluster being readonly and not being to elect a new leader in
some situations because of pre-vote.
Connection death timeout is 4*replication_timeout for all nodes.

'strict' - fencing turned on. In this mode leader tries its best to
resign leadership before new leader can be elected. This is achived
by halving death timeout on leader.
Connection death timeout is 4*replication_timeout for followers and
2*replication_timout for current leader.