Support running KRPC over Unix domain socket

KRPC was extended to support Unix domain socket in Apache Kudu. The patches for this new feature involves code change in kudu/rpc, kudu/security and kudu/util. We recently synced up the source code in kudu/rpc, kudu/security and kusu/util with Kudu upstream so we can leverage the new functionalities provided by Kudu source to support running KRPC over Unix domain socket.
For the Impala mini-cluster, all Impala daemons are running on the same host. We can run KRPC over Unix domain sockets instead of TCP.