Implement in Java the CachedImmutableBucket that exists in Python lib #3

Python lib contains CachedImmutableBucket class.

Please implement the same class for Java lib, and publish to Maven.

The implementation probably would require the implementation of AppendOnlyFSBucket, and FileLockForPath.
The FileLockForPath is already implemented in this PR: #1
but it's not verified/well tested.
As first step, please finish and merge the FileLockForPath, and as second step, implement the CachedImmutableBucket