Add thriftmux support in finagle-clojure/thriftmux

[ThriftMux](http://twitter.github.io/finagle/docs/index.html#com.twitter.finagle.ThriftMux$)
serves Thrift using the
[Mux](http://twitter.github.io/finagle/docs/index.html#com.twitter.finagle.mux.package)
protocol.

See http://twitter.github.io/finagle/guide/Protocols.html#mux for more
info on Mux.

Mux is a session layer RPC multiplexer. It offers many advantages over
plain Thrift such as fewer connections (one per client process rather
than one per client request) and session liveliness detection.