gmirror: Evaluate mirror components against newest metadata copy

gmirror: Evaluate mirror components against newest metadata copy

If we happen to taste a stale mirror component first, don't reject valid,
newer components that have differing metadata from the stale component
(during STARTING).  Instead, update our view of the most recent metadata as
we taste components.

Like mediasize beforehand, remove some checks from g_mirror_check_metadata
which would evict valid components due to metadata that can change over a
mirror's lifetime.  g_mirror_check_metadata is invoked long before we check
genid/syncid and decide which component(s) are newest and whether or not we
have quorum.

Before checking if we can enter RUNNING (i.e., we have quorum) after a NEW
component is added, first remove any known stale or inconsistent disks from
the mirrorset, rather than removing them *after* deciding we have quorum.
Check if we have quorum after removing these components.

Additionally, add a knob, kern.geom.mirror.launch_mirror_before_timeout, to
force gmirrors to wait out the full timeout (kern.geom.mirror.timeout)
before transitioning from STARTING to RUNNING.  This is a kludge to help
ensure all eligible, boot-time available mirror components are tasted before
RUNNING a gmirror.

When we are instructed to forget mirror components, bump the generation id
to avoid confusion with such stale components later.

Add a basic test case for STARTING -> RUNNING startup behavior around stale
genids.

PR:		232671, 232835
Submitted by:	Cindy Yang <cyang AT isilon.com> (previous version)
Reviewed by:	markj (kernel portions)
Discussed with:	asomers, Cindy Yang
Tested by:	pho
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D18062