[SAMBA4] highestCommitedUSN support

[tridge at samba.org]

Metze,

 > I had the idea for maintain the highestCommitedUSN seperate from the seqnumber...

we could do, but it would mean re-writing the rootdse record on each
add/modify, in addition to the re-writing of the @BASEINFO record. I'm
concerned about the overhead of that. That's why I did
highestCommitedUSN as a generated attribute, much like the currentTime
field, except that it comes from the @BASEINFO record which ldb_tdb is
already maintaining.

Is there something else that maintaining it separately would gain us?

Cheers, Tridge