Bugfix release

Bugfixes:

• Fix CancelledError handling in sender (issue #710)
• Fix exception for weakref use after object deletion (issue #755)
• Fix consumer's start() method hanging after being idle for more than
  max_poll_interval_ms (issue #764)

Improved Documentation:

• Add SASL_PLAINTEXT and SASL_SSL to valid values of security protocol
  attribute (pr #768 by @pawelrubin)