Steady state detection: enhance reporting of results, change memory allocation point

1. Always record and report both BW and IOPS for steady state window (useful when multiple block sizes are used)
2. Always report both slope and maximum deviation even though one is always unused
3. Report mean BW and mean IOPS in JSON output
4. Allocate memory for ring buffers in steadystate_setup() instead of add_job(). The benefit is that only one set of buffers is now allocated for eaceh group. Previously each thread had its own set of buffers enough though only one set in each reporting group was actually used.
5. Update test script to accommodate new output