io_tester: move execution code to a thread.
io_tester: move execution code to a thread.

The code that drives the test is now in a bunch of futures. It's not that bad,
but upcoming changes will make it significantly less readable.

Since this is only firing the execution and is not performance critical make
it a seastar::thread instead.

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

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