In some cases ActiveSession.cleanup was not called after authentication,
so for some user ActiveSession lookup keys grew without ever cleaning
up. This Rake task manually iterates over the lookup keys and removes
ones without existing ActiveSession.
By doing smaller mget calls to Redis, it can better schedule the
workload. Currently a single mget with a lot of keys can keep Redis busy
for long, while nothing in its queue gets processed.
Adds ActiveSession#session_ids_for_user for listing session IDs for a
given user, and adds ActiveSession#list_sessions for listing session
data directly.
