Raft cluster reconfigiration consists of two parts, first leader has to
consent on configuration block, next leader has to extract new cluster
configuration and propose raft configuration changes. However leader
might fail between first and second parts, therefore newly selected
leader should be able to detect there is unfinished reconfiguration and
to finish reconfiguration.

This commit adds logic to manage leadership failover, where new leader
checks whenever last committed block is configuration block, whenever
there are pending configuration changes and complete reconfiguration by
proposing raft clust configuration changes.

Change-Id: I05dc1f60c9ab692521887b50f726d96ea47878dc
Signed-off-by: Artem Barger <bartem@il.ibm.com>