logging: record timestamp for each thread

logging: record timestamp for each thread

Instead of recording a timestamp once before iterating through all the
threads to check if we should add a new bw or iops log measurement,
record a new timestamp before checking each thread.

We were already querying the time anyway for the mtime_since_now() call.
We might as well have it available locally for more accurate logging.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>