LU-12168 utils: obdfilter fix for SHORT msgs

LU-12168 utils: obdfilter fix for SHORT msgs

Sometimes obdfilter-survey shows SHORT instead of min,max.
This could happen when two signals for a parent process comes
during a verbose time. The counters are updated and start_time
is dropped. By default timeperiod is 1 second.

ost  1 sz 16777216K rsz 2048K obj    4 thr    8
write 3662.99 [4286.00,4528.95] rewrite 3873.87 [4746.85, 4857.48]
read 8088.39      SHORT

The patch fixes this issue and drops counters and time when
statistics are printed or all threads are started.

Obdfilter-survey can print SHORT after patch when subtest time
is too small 1-2 seconds. The detail log shows this case as

Total: total 8192 threads 4 sec 1.692006 4841.590396/second

Test-Parameters: trivial
Signed-off-by: Alexander Boyko <c17825@cray.com>
Cray-bug-id: LUS-7110
Change-Id: I9b1521c23e9360216a279ab5c28c39bcaca9974b
Reviewed-on: https://review.whamcloud.com/34610
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>