Fix Findbugs Warnings

The findbugs complained a lot of warnings after upgrade:
https://builds.apache.org/job/PreCommit-ZOOKEEPER-Build/2191//artifact/trunk/build/test/findbugs/newPatchFindbugsWarnings.html#Warnings_I18N

It would be great to get those warnings settled before 3.5.0 release.

My proposal is:
1. Ignore those "Internationalization Warnings" which is related to encoding. And create a new JIRA to fix encoding later.
2. fix warnnings of:

• Multithreaded correctness Warnings
• Performance Warnings
• Dodgy code Warnings