Introduce a skipIntermediate feature for zfs sending faster and smaller chunks

[jimklimov]

Follows up from @lotheac 's #455 and https://github.com/lotheac/znapzend/pull/1

UPDATE: I believe this also achieves at least partially the declared goals of some currently open PRs:

• 230 wants to avoid a blanket send -I with a filter routine to construct the list of snapshots we want on the destination and only step through them (it might also be a useful fit for evolution of #457); CC @renarded

• 314 should be completely covered by this PR, as its change is identical to #455 that triggered it (and the pros/cons discussion is quite similar); CC @znek

[coveralls]

Coverage increased (+0.02%) to 90.094% when pulling 01850dda3ecb5475ad482199f7bf69d240ded9c2 on jimklimov:intermediate-feature into c54478c9bffde5d254ac75959841aabf11e21bcb on oetiker:master.

[coveralls]

Coverage decreased (-0.04%) to 90.033% when pulling 01850dda3ecb5475ad482199f7bf69d240ded9c2 on jimklimov:intermediate-feature into c54478c9bffde5d254ac75959841aabf11e21bcb on oetiker:master.

[jimklimov]

@oetiker : This one seems good, no not ok and done_ hits found in Travis build logs :)