Replicate counter identity rules

[neongreen]

Looks like we do not need to remove counters when rules are reloaded. The behavior of lyft/ratelimit is as follows:

• When rules are reloaded, existing counters are not touched.
• When the reloaded rules are the same but one limit has changed, the counter is not reset, and the new limit is taken into account.
• When the reloaded rules are the same but one unit has changed, a new counter is used, and the old counter is not removed.

So we need to:

• add ratelimit unit to the CounterKey
• document in code that counters don't need to be removed, and explain why
• document the behavior in README/elsewhere
• write tests