commit log and heartbeat issue

[xiang90]

When doing committing, the log is locked. This is a disk operation, and sometime it may take a longer time.
During that operation, our current implementation cannot do heartbeat. This will delay heartbeat and leads to election timeout.

[benbjohnson]

@xiangli-cmu We probably need to switch the locks to use sync.RWMutex. That should fix the problem.