Introduce minimal ZIL block commit delay

Despite all optimizations, tests on actual hardware show that FreeBSD
kernel can't sleep for less then ~2us.  Similar tests on Linux show
~50us delay at least from nanosleep() (haven't tested inside kernel).
It means that on very fast log device ZIL may not be able to satisfy
zfs_commit_timeout_pct block commit timeout, increasing log latency
more than desired.

Handle that by introduction of zil_min_commit_timeout parameter,
specifying minimal timeout value where additional delays to aggregate
writes may be skipped.  Also skip delays if the LWB is more than 7/8
full, that often happens if I/O sizes are constant and match one of
LWB sizes.  Both things are applied only if there were no already
outstanding log blocks, that may indicate single-threaded workload,
that by definition can not benefit from the commit delays.

While there, add short time moving average to zl_last_lwb_latency to
make it more stable.

Tests of single-threaded 4KB writes to NVDIMM SLOG on FreeBSD show IOPS
increase by 9% instead of expected 5%.  For zfs_commit_timeout_pct of
1 there IOPS increase by 5.5% instead of expected 1%.

Reviewed-by: Allan Jude <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Prakash Surya <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #14418 

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