Ensure that all memory returned by the allocator is 8 byte aligned.

Don't needlessly track full Slices. They are are located by the
Slice Allocator when an allocation is released from the Slice
and then placed back into the partial list. This saves
a small amount of list manipulation.
Correct underflow time issue that causes unnecessary Slice and Memory
pool releases during the first 120 seconds of operation.
Revert timing test change.
Merge branch 'master' of github.com:openzfsonosx/spl