reader_concurrency_semaphore: rearrange members
reader_concurrency_semaphore: rearrange members

Use standard convention of the rest of the code base. Type definitions
first, then data members and finally member functions.
As we are about to add more members, its especially important to make
the growing class have a familiar member arrangement.

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