Resolve IS2_INCONSISTENT_SYNC findbugs warning in AuthenticationTokenSecretManager

After efforts in HBASE-15118, we still see IS2_INCONSISTENT_SYNC warning in AuthenticationTokenSecretManager in HadoopQA report for HBASE-13960:

In class org.apache.hadoop.hbase.security.token.AuthenticationTokenSecretManager
Field org.apache.hadoop.hbase.security.token.AuthenticationTokenSecretManager.lastKeyUpdate
Synchronized 50% of the time
Unsynchronized access at AuthenticationTokenSecretManager.java:[line 343]
Synchronized access at AuthenticationTokenSecretManager.java:[line 278]

Checking the code, we could see synchronized (this) in line 343 is synchronizing on the instance of the subclass AuthenticationTokenSecretManager$LeaderElector while lastKeyUpdate is a variable of the parent class instance

Will fix the issue in this JIRA