Addition of new ZooTrace Mask LEADER_ELECTION_MASK and related log lines for better leader election telemetry

Recently we have been seeing longer leader election abnormalities and we don't have visibility into "why" leader election took this long and "where exactly" it took long. As a result we cannot explain the long Zk service outages which we're seeing. To handle this I created a change that adds:

final static public long LEADER_ELECTION_MASK = 1 << 10; in ZooTrace.java

In order to add additional TRACE level logs at various shutdown calls. Is this new mask alright to add? If not, what are some alternatives to this change?

PR here: https://github.com/apache/zookeeper/pull/1583