semaphore: provide API to query the number of waiters
semaphore: provide API to query the number of waiters

This will let us know how busy a given semaphore is. By doing this here,
we can simplify callers since they won't have to track this.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

kjniemi/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,dreamsxin/seastar,sjperkins/seastar,dreamsxin/seastar,sjperkins/seastar,avikivity/seastar,sjperkins/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,kjniemi/seastar,cloudius-systems/seastar,kjniemi/seastar,scylladb/seastar