Thrift connection pooler breaks if thrift service inheritance is used

The thrift connection pooler in desktop/core/src/desktop/lib/thrift_util.py keys its dictionary of client connections by (host, port) pair. If a service which Hue is communicating with utilizes thrift's service inheritance feature, it's potentially possible for multiple distinct thrift service interfaces to be available on a single (host, port) pair. The thrift connection pooler should therefore key by (client class, host, port) three tuple.