config: add failover stateboard options

These options configure how often a state information is updated by the
failover coordinator in a remote state storage and how fast a transient
state is expired.

The new options are named as follows.

* `failover.stateboard.renew_interval`
* `failover.stateboard.keepalive_interval`

Part of https://github.com/tarantool/tarantool-ee/issues/564

NO_DOC=to be added separately
NO_CHANGELOG=Enterprise Edition feature, no new behavior in the
             Community Edition