support for async TSaslClientTransport

[fireyyouth]

• background
  Our team used to rely on official python thrift client to access big data services (i.e. hbase, kafka, hive), through sasl authentication.
  We are planning to make everything async and thriftpy2 appears very suitable to our need af first glance.
• problem
  there is a TSaslClientTransport class for sasl authentication, but can't be used in async mode.

1. There is no example of how to use TSaslClientTransport , please provide one
2. Is there any plan to support async version of of TSaslClientTransport ?
3. Is there any workaround to make async and sasl work together?