THRIFT-1914 Python: Support for Multiplexing Services on any Transport

copy patch from: https://issues.apache.org/jira/browse/THRIFT-1914 add some testcases.

add new options multiple, if set True, will test ThriftTest and SecondService.

run server:

    $ python TestServer.py --port=9999 --multiple -v TSimpleServer

run client:

    $ python TestClient.py --host=127.0.0.1 --port=9999 --multiple
    ...................
    ----------------------------------------------------------------------
    Ran 19 tests in 0.693s

    OK

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