queue: add has_blocked_consumer() function
queue: add has_blocked_consumer() function

Sometimes it is useful for a producer to know if someone waits for an
item to be produced and expedite its creation if someone does. Add
has_blocked_consumer() function to facilitate such use.

Message-Id: <14094c6194ff8e7a58335751ca7ce716735276c7@scylladb.com>

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