"
Current admission control takes a permit when cql requests starts and
releases it when reply is sent, but some requests may leave background
work behind after that point (some because there is genuine background
work to do like complete a write or do a read repair, and some because
a read/write may stuck in a queue longer than the request's timeout), so
after Scylla replies with a timeout some resources are still occupied.
The series fixes this by passing the permit down to storage_proxy where
it is held until all background work is completed.
Fixes#4768
"
* 'gleb/admission-v3' of github.com:scylladb/seastar-dev:
transport: add a metric to follow memory available for service permit.
storage_proxy: store a permit in a read executor
storage_proxy: store a permit in a write response handler
Pass service permit to storage_proxy
transport: introduce service_permit class and use it instead of semaphore_units
transport: hold admission a permit until a reply is sent
transport: remove cql server load balancer
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