io_tester: Prepare for rps parameter
io_tester: Prepare for rps parameter

Introduce a new "rps" option in a job config. The parameter
will denote how many requests per second will need to be sent
to a disk.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar