TACHYON-96 - Change AbstractTachyonFSCore abstract class to TachyonFSCore interface.

I would like to make AbstractTachyonFSCore to be an interface instead:
1. Since AbstractTachyonFSCore does not implement any method, as recommended in effective Java it would be preferable to make it an
interface.
2. For testing purposes, having abstract class does not help to have mock but an interface does.
3. Java class can only extend from one class but can implement multiple interfaces.

This would help pluggability plan that we have for Tachyon APIs.
Remove public accessor for the interface methods.
Interface methods are public by default.
Update Javadoc.
Change fid to fileId to be consistent.
Merge pull request #390 from hsaputra/TACHYON-96_make_interface_for_TAchyonFS

TACHYON-96 - Change AbstractTachyonFSCore abstract class to TachyonFSCore interface.