1. When the cache is full, a miss evicts the most idle factory.
When no underlying factories are idle, a one-shot factory is created.
This doesn't necessarily guarantee good performance: one-shots could
be created constantly for a hot cache key, but should work well
when there are a few hot keys.
2. Periodically evict factories that are idle for at least one TTI
(time-to-idle) period. An idle factory could remain in the cache
for up to (TTI * 2) minutes, with the caveat that we never expire
the last, least-idle entry.
A "read-through" cache of service factories.
Eviction is based on cache size and idle time:
1. When the cache is full, a miss evicts the most idle factory. When no underlying factories are idle, a one-shot factory is created. This doesn't necessarily guarantee good performance: one-shots could be created constantly for a hot cache key, but should work well when there are a few hot keys.
2. Periodically evict factories that are idle for at least one TTI (time-to-idle) period. An idle factory could remain in the cache for up to (TTI * 2) minutes, with the caveat that we never expire the last, least-idle entry.