MFV r318931: 8063 verify that we do not attempt to access inactive txg

illumos/illumos-gate@b7b2590dd9f11b12a0b4878db3886068cce176af
https://github.com/illumos/illumos-gate/commit/b7b2590dd9f11b12a0b4878db3886068cce176af

https://www.illumos.org/issues/8063

A standard practice in ZFS is to keep track of "per-txg" state. Any of
the 3 active TXG's (open, quiescing, syncing) can have different values
for this state. We should assert that we do not attempt to modify other
(inactive) TXG's.

Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>

MFC after: 2 weeks