Improve on InMemoryStorage proposals

[alexsnaps]

Adding this as a dumping ground for ideas of possible improvements we could implement to make InMemoryStorage more memory efficient and made more parallel:

• Have Counters sharable across threads #69
• "variadic" counters with interior mutability? (so to only grow the Set under write lock)
• Get rid of the TtlCache<Counter, i64> and store counters in something like NonBlockingCounter (see InMemoryStore grows unbounded #71)
• Possibly avoid duplication of data (e.g. Counter.limit, have Limit being composed of Counters instead)… i.e. remodel things to match the reality of the domain, not the persistence of it. Possibly relates to Redefine authority of Limits #74
• Should we make this a hierarchical storage? One where counters would always sit in memory, certainly for short-lived counters (seconds/minutes, possibly configurable) where persistence make little sense