This PR introduces a new testonly endpoint for introspecting the
RequestLimiter state. It makes use of the endpoint to verify that changes to
the request_limiter config are honored across reload.
In the future, we may choose to make the sys/internal/request-limiter/status
endpoint available in normal binaries, but this is an expedient way to expose
the status for testing without having to rush the design.
In order to re-use as much of the existing command package utility funcionality
as possible without introducing sprawling code changes, I introduced a new
server_util.go and exported some fields via accessors.
The tests shook out a couple of bugs (including a deadlock and lack of
locking around the core limiterRegistry state).
