Allow overriding provision of FileSystem instances to HadoopTableOperations

[mccheah]

Util.getFS forces the FileSystem objects to be cached at the JVM level using FileSystem.get. In specific situations, sometimes one would prefer finer-grained control over the caching of these objects.

We propose the following:

• Add protected FileSystem getFS(Path path, Configuration configuration) in HadoopTableOperations. Default implementation calls Util#getFS.
• Make HadoopTableOperations public with a protected constructor so that users can use some of the behavior but override specific components, particularly File I/O Submodule for TableOperations #12 and the FileSystem provision change above.