OpenZFS 8600 - ZFS channel programs - snapshot

OpenZFS 8600 - ZFS channel programs - snapshot

Authored by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Don Brady <don.brady@delphix.com>
ZFS channel programs should be able to create snapshots.
In addition to the base snapshot functionality, this entails extra
logic to handle edge cases which were formerly not possible, such as
creating then destroying a snapshot in the same transaction sync.

OpenZFS-issue: https://www.illumos.org/issues/8600
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/68089b8b