new TSDB constructor with initialized hbase client

Hi Chris, Hi Benoît,

When using OpenTSDB as a java lib, we sometimes need to get several instances of TSDB in the same application, or to use the HBaseClient directly from the same application.

From AsyncHBase readme :
>Unlike HTable, you should have only one instance of HBaseClient in your application, regardless of the number of tables or threads you want to use