This pull request adds partial admission control to Thrift frontend. The solution is partial mostly because the Thrift layer, aside from allowing Thrift messages, may also be used as a base protocol for CQL messages. Coupling admission control to this one is a little bit more complicated due to how the layer currently works - a Thrift handler, created once per connection, keeps a local `query_state` instance for the occasion of handling CQL requests. However, `query_state` should be kept per query, not per connection, so adding admission control to this aspect of the frontend is left for later.
Finally, the way service permits are passed from the server, via the handler factory, handler and then to queries is hacky. I haven't figured out how to force Thrift to pass custom context per query, so the way it works now is by relying on the fact that the server does not yield (in Seastar sense) between having read the request and launching the proper handler. Due to that, it's possible to just store the service permit in the server itself, pass the reference (address) to it down to the handler, and then read it back from the handling code and claim ownership of it. It works, but if anyone has a better idea, please share.
Refs #4826Closes#8313
* github.com:scylladb/scylla:
thrift: add support for max_concurrent_requests_per_shard
thrift: add metrics for admission control
thrift: add a counter for in-flight requests
thrift: add a counter for blocked requests
thrift: partially add admission control
service_permit: add a getter for the number of units held
thrift: coroutinize processing a request
memory_limiter: add a missing seastarx include
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