Reuse hs2-http connections in impala-shell

Currently, impala-shell creates a new connection for every request when running the hs2-http protocol. The reason for this is that we're using the default Thrift python HTTP client which is very basic and doesn't reuse connection for simplicity. It would be nice to reuse connections for performance reasons.

We've already imported the code for it into Impala to make other changes, so it should be easy to further modify it to support connection reuse.

See: https://github.com/apache/impala/blob/master/shell/ImpalaHttpClient.py#L160