Introduces the rate_limiter, a replica-side data structure meant for
tracking the frequence with which each partition is being accessed
(separately for reads and writes) and deciding whether the request
should be accepted and processed further or rejected.
The limiter is implemented as a statically allocated hashmap which keeps
track of the frequency with which partitions are accessed. Its entries
are incremented when an operation is admitted and are decayed
exponentially over time.
If a partition is detected to be accessed more than its limit allows,
requests are rejected with a probability calculated in such a way that,
on average, the number of accepted requests is kept at the limit.
The structure currently weights a bit above 1MB and each shard is meant
to keep a separate instance. All operations are O(1), including the
periodic timer.
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