reader_concurrency_semaphore: add name() accessor
reader_concurrency_semaphore: add name() accessor

Allows identifying the semaphore in question in semaphore related error
messages.

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