Fixed deadlock in producer.send_async

[merlimat]

Fix #84

Release the GIL while calling producer.sendAsync() to avoid a deadlock when PyBind is triggering the Python callback.

• Main Thread

  1. Holds the Python GIL
  2. Call producer.send_async()
  3. Tries to acquire internal ClientConnetion lock
  • Pulsar client internal thread
    1. Holds lock on ClientConnection
    2. Receives ack from the broker
    3. Triggers callback
    4. PyBind11 acquires GIL <---- Deadlock

The problem is the different behavior in PyBind from Boost::Python.

We always need to make sure we release the GIL before making any call to C++ that potentially acquires any mutexes