io_tester: fixes affecting throughput issuing/calculating
io_tester: fixes affecting throughput issuing/calculating

sleep will always break a continuation. When think_time is zero, the
behavior we want is to fire right away, with no breaking.

We also shouldn't account for requests that arrive after the clock
stopped.

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

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