reader_concurrency_semaphore: expose signal/consume
reader_concurrency_semaphore: expose signal/consume

To allow the amount of available resource to be adjusted after creation.

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla