[SAMBA4] highestCommittedUSN support

[tridge at samba.org]

Luke,

 > Right, but when you startup you can just search for all entries where
 > usnChanged > highestCommittedUSN, and then update the in-memory NC
 > high watermark.

ahh, unfortunately we can't do that style of index (greater than) in
ldb yet, so we'd have to do a full scan of the db. With the approach
we're taking in ldb of attaching to the database separately in many
processes, this could get pricey. There might be some approach that
could work, but I think we'll stick to the @BASEINFO sequenceNumber
for now :)

Cheers, Tridge