Redis result backend: Adds support for a max_connection parameter.
Redis result backend: Adds support for a max_connection parameter.

This enables the possibility to configure the maximum number of
simultaneous connections used for results in the redis connection pool.

The default max connections setting can be configured using the
:setting:`CELERY_REDIS_MAX_CONNECTIONS` setting,
or it can be changed individually by ``RedisBackend(max_connections=int)``.

Closes #467

cbrepo/celery,ask/celery,ask/celery,cbrepo/celery