limiter API revision, part 1/2 - channel-side API
limiter API revision - part 2/2 - queue-side API
correctness++
niggles--
some documentation
always hold a valid limiter in queue's channel records
This is less fiddly, but does mean we have to pass the limiter pid in
basic_get.
better names for the prefetch related part of the limiter API
Merge in default
Merge bug25461