Make HTable accesses thread-safe

Fixes IBMStream/streamsx.hbase #37. 

The  problem is that this fix requires hbase 0.94.11 or better.  So, I added the option of putting jars in opt/local_override that supersede whatever is in HBASE_HOME.  That way, if you're HBASE_HOME is at 0.94.3, then you can get an newer HBASE jar, put it in opt/local_override. 

I will leave this pull request open for at least 24 hours.