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

TACHYON-96 - Change AbstractTachyonFSCore abstract class to TachyonFSCore interface.

Looks like TachyonFS now extends AbstractTachyonFS which extends AbstractTachyonFSCore.
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.
Might be better to call this FileSystem instead of Core.  This change is user-facing, so the name should make sense to users.

`TachyonFileSystem` or `FileSystem` (i prefer the second).