HBase Backup/Restore Phase 3: Create plugin hooks for Backup tools

Different organizations may already have their own backup tools for HBase, or wish to develop them in the future for their own particular use cases. It would be useful if HBase supported hooks to integrate those tools so that they could be configured and run in a standard way.

In particular, the administrative interface to start a backup, restore, or merge should be decoupled from any particular implementation as much as possible, so that any implementation with similar capabilities can be substituted via configuration without needing to fork or modify the code.

Ideally, it will also be possible to decouple the various components so that implementations can be mixed and matched. (For example, one could use the standard backup's functionality to track what needs to be backed up, but use a custom plugin to change the logic or storage format of the backup operation itself.)