[SAMBA4] highestCommitedUSN support

[tridge at samba.org]

Andrew,

 > One of the early tests is for the presence of the highestCommitedUSN
 > attribute.  I understand that the ldb_tdb backend already supports
 > something like this, so I'm trying to find out how best to access it.
 > Having the rootdse module fetch the @BASEINFO record and sequenceNumber
 > attribute seems wrong...

Right, sequenceNumber in @BASEINFO is a pretty good match for
highestCommitedUSN.

 > Likewise, it would seem the correct opportunity to have each record
 > marked with it's correct sequence number.

yep, probably the operational module is the best fit for this, as its
a natural extension of the various timestamp fields that we handle now.

 > We probably also need to store suitable details for syncRepl, if we
 > are every going to support that.

I haven't looked into whats needed for that
 
 > Anybody feel like coding this up, or at the very least providing some
 > hints on how to do sanely?

I'll have a go at this unless you beat me to it.

Cheers, Tridge