Set TTL to expire keys instead of relying to Redis eviction policy

[agis]

Right now, we implicitly require administrators to have Redis configured with maxmemory and maxmemory-policy set to allkeys-lru. While this is convenient for us, it's not very bullet-proof since we don't guarantee that the timings key will not be evicted somehow. For example, the Redis instance might be reaching maxmemory for other reasons and cause the timings key to be evicted, if no builds have run for some time and some other app uses the same instance for some reason.

Instead of relying on this configuration, we could explicitly set TTLs to all the keys we use, except of those that should be persisted, i.e. timings.

This probably obsoletes #5