Improve log spacemap load time

Previous flushing algorithm limited only total number of log blocks to
the minimum of 256K and 4x number of metaslabs in the pool. As result,
system with 1500 disks with 1000 metaslabs each, touching several new
metaslabs each TXG could grow spacemap log to huge size without much
benefits. We've observed one of such systems importing pool for about
45 minutes.

This patch improves the situation from five sides:

• By limiting maximum period for each metaslab to be flushed to 1000

TXGs, that effectively limits maximum number of per-TXG spacemap logs
to load to the same number.

• By making flushing more smooth via accounting number of metaslabs

that were touched after the last flush and actually need another flush,
not just ms_unflushed_txg bump.

• By applying zfs_unflushed_log_block_pct to the number of metaslabs

that were touched after the last flush, not all metaslabs in the pool.

• By aggressively prefetching per-TXG spacemap logs up to 16 TXGs in

advance, making log spacemap load process for wide HDD pool CPU-bound,
accelerating it by many times.

• By reducing zfs_unflushed_log_block_max from 256K to 128K, reducing

single-threaded by nature log processing time from ~10 to ~5 minutes.

As further optimization we could skip bumping ms_unflushed_txg for
metaslabs not touched since the last flush, but that would be an
incompatible change, requiring new pool feature.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12789