Fix javadoc and checkstyle issues in timelineservice code

There are a large number of javadoc and checkstyle issues currently open in timelineservice code. We need to fix them before we merge it into trunk.

Refer to https://issues.apache.org/jira/browse/YARN-3862?focusedCommentId=15035267&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15035267
We still have 94 open checkstyle issues and javadocs failing for Java 8.