--active-only expensive when many queues are active, add caching option

[ryanwitt]

After switching to --active-only on jobs that have a large number of dynamic queues, we notice that we start spending a lot of money on GetQueueAttributes calls:

This makes sense, comparing the --active-only API call complexity with the base case, when a is high, then so are the calls:

Context	Calls	Details
qdone worker (while listening, per listen round)	n + (1 per n×w)	w: --wait-time in seconds n: number of queues
qdone worker (while listening with --active-only, per round)	2n + (1 per a×w)	w: --wait-time in seconds a: number of active queues

However the state of the active queues is very cacheable, especially if queues tend to have large backlogs, as ours do.

I propose we add three options:

• --cache-url that takes a redis://... cluster url [no default]
• --cache-ttl-seconds that takes a number of seconds [default 10]
• --cache-prefix that defines a cache key prefix [default qdone]

The presence of the --cache-url option will cause the worker to cache GetQueueAttributes for each queue for the specified ttl. Probably can use mget for this, if we're careful about key slots.