Flush unused buckets out of memory

[shtlrs]

The rate limiting solution uses the process' memory to store the token buckets.

If for some reason, the service becomes popular and a lot of clients target it, the number of buckets will start to grow, and so does the memory consumption.

I think that we should timestamp the buckets with the time they were last used, and have a background job like reap that runs every N seconds to flush the stale buckets out of memory.