OpenZFS 7230 - add assertions to dmu_send_impl() to verify that stream includes BEGIN and END records

Authored by: Matt Krantz <matt.krantz@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: kernelOfTruth <kerneloftruth@gmail.com>

OpenZFS-issue: https://www.illumos.org/issues/7230
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/12b90ee2
Closes #5112