Initial riak support
Add some documentation on class attributes

Also add handling of protocol.
Cleanup code related to connection to Riak

This is a leftover of previous work on this backend.
Switch default bucket name to celery
Add riak to extra requirements
Add Riak backend documentation
Fixed error in protocol handling for the riak backend
Use a heapq to avoid calling all entries for every iteration
Remove print
Tox: Add debug test output
Tox: Set environment variables properly
Make the database backend retry operations on ResourceClosedError and StaleDataError too. Make the operations close the connection if failure occurs (can't retry on broken connection). Fixes #1786.
Don't close the session. Just rollback and/or close.
Close the sessions before disposing the engines as the engines won't close connections held up in sessions.
Use a session manager that has different behavior before the fork (effectivelly hardcodes NullPool - everything else is unreliable).
Update changelog.
Merge pull request #1899 from celery/more-reliable-sqlalchemy

Make the database backend more reliable
Cosmetics for #1736
flakes
Stress test app now loads on Windows