fio: steadystate: allow for custom check interval

fio: steadystate: allow for custom check interval

Allow for a different steady state check interval than 1s with
a new --ss_interval parameter.

Steady state is reached when the steady state condition (like slope) is
true when comparing the last windows (set with --ss_dur).
The actual values for this comparison is currently calculated for a 1s
interval during the window.
This is especially problematic for slow random devices, where the values do
not converge for such a fine granularity.
Letting the user set this solves this problem, although requires them figuring
out an appropriate value themselves.

--ss=iops:5% --ss_dur=120s should reproduce this for many (slower) devices.
Then adding like --ss_interval=20s may let it converge.

Signed-off-by: Christian Loehle <cloehle@posteo.de>