io queue: don't allow more than 128 requests concurrently
io queue: don't allow more than 128 requests concurrently

Even if the I/O Scheduler has more capacity than that, requests may
block if they are sent down the reactor, since the reactor itself can
only hold up to 128 requests concurrently. I intend to unify those
things eventually, so we'll do that by enforcing that limit in the I/O
Scheduler queues as well.

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

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