log: fix timestap precision of log can't set to millisecond.

log: fix timestap precision of log can't set to millisecond.

The option log_coarse_timestamps can be set to Log::clock
successfully,but the Log::clock has no effect on time accuracy
because the dout_impl really use is Entry::clock.So we should
set Entry::clock by log_coarse_timestamps option instead of
Log:clock.In addition, i think the Log::clock can be removed
because i didn't see what it was for.

Fixes: https://tracker.ceph.com/issues/44409
Signed-off-by: Guan yunfei <yunfei.guan@xtaotech.com>