Nop write (WIP)

This pull request is for review purposes only.  After applying the upstream nop-write changes I'm now able to easily reproduce the following upstream issue, https://www.illumos.org/issues/3113.  I'm fairly certain this change doesn't introduce the problem it just makes it more likely to occur.  Regardless, these commits cannot be merged until the ztest issue is resolved.  I'm posting my work in the hopes that someone else might have some time to look in to it.