Adding support for Phoenix

Managed to add a plugin store for Phoenix to run linkbench on top of hbase through Phoenix.

This is the initial version. unit tests added and passed. While since there are limitations of Phoenix, many SQL feature not supported yet. A few more features need to be fixed later on phoenix side or found a better work around to improve performance and ensure correctness (mainly for "count").

Also, tweak a little bit code other than phoenix store, mainly for 'data' field generation, in order to make sure the NOT NULL limitation on filed could be guaranteed.

Please help to take a review ;)