ACCUMULO-2558 Add unit tests for server/gc

Code changes accompanying unit tests:
- visibility of methods under test changed from private to package
- some methods refactored to take arguments, to permit testability
- GarbageCollectWriteAheadLogs.isUUID() improved to check for null and to check
  length of potential UUID, since UUID.fromString() will accept UUIDs with extraneous
  leading zeroes in components
- SimpleGarbageCollector.getZooLock() finds ZK root using instance field of class
  instead of calling HdfsZooInstance.getInstance() again
- SimpleGarbageCollector.init() no longer declares throwing IOException