MFC r344934, r345014: Add separate aggregation limit for non-rotating media.

Before sequential scrub patches ZFS never aggregated I/Os above 128KB.
Sequential scrub bumped that to 1MB, which motivation I understand for
spinning disks, since it should reduce number of head seeks. But for
SSDs it makes much less sense to me, especially on FreeBSD, where due
to MAXPHYS limitation device will likely still see bunch of 128KB I/Os
instead of one large. Having more strict aggregation limit allows to
avoid allocation of large memory buffer and memcpy to/from it, that is
a serious problem when bandwidth reaches few GB/s.

Sponsored by: iXsystems, Inc.