multi dump support for storage system classes

most of the files in this PR have already been reviewed. 
I had to add a connection_context to the hbase classes to better support threading.  The following files need reexamination:
     socorro/external/hbase/connection_context.py
     socorro/unittest/external/hbase/test_connection_context.py
     socorro/external/hbase/crashstorage.py
     socorro/unittest/external/filesystem/test_crashstorage.py