HUE-1110 [impala] Support Hive Server 2 API

In order to use HS2 interface with Impala, update [impala]
server_port=21050.
Beeswax sessions are now differentiated among Beeswax and Impala
(that way both sessions do not conflict).
Bunch of little fixes for supporting both HS2 Hive/Impala apis.