Fix to compiler error on Mac (eBay#221)

[Refactor] Rename member variables (eBay#174)

Add new callback event for new session from leader (eBay#115)

* We should differentiate opening connection and receiving message
from valid leader, as leader can be changed without closing connection.

Fix a place where sm_commit_index_ is wrongly updated (eBay#94)

Allow election timer on restart_election_timer API (eBay#82)

* Election timer flag should be updated on election timer restart,
as the flag will not be effective anymore if timer is initiated.

Handle the case when logs do not exist (eBay#37)

* If there is no snapshot, and logs are already compacted, leader
should protect itself rather than crashing.

* Update logger to fix recursive crash handler issue.