An attempt to fix #642.

Bug recap

When using Sentinel, in get_scheduler_statistics function, queue.connection.connection_pool.connection_kwargs does not have a host key, making the function to raise a KeyError exception when it tries to define conn_key based on connection['host']. This makes the /admin/background-tasks page to return a 500 error, making it unusable.

What does this fix do

This PR fixes this by building conn_key from all sentinels host:port pairs.

With this fix, a scheduler configured with Sentinel appears like this (with mymaster being the Sentinel Service, and 0 the configured db index for this scheduler):

Sentinel(sentinel-1:26379,sentinel-2:26379,sentinel-3:26379)://mymaster/0

If you don't agree with this syntax, please let me know and I change it.

Alternative solution

An alternative solution would be to build the conn_key from the current Sentinel master for the given service, but I'm not sure what you prefer. It would make the code a bit easier to read, and would make the resulting string in the UI much shorter. But it's less clear about the config used for this scheduler.

NeodymiumFerBore/django-rq#2 implements this solution, if you want to take a look...