s4-ldb: ldb indexing rewrite - part1

This gets rid of the @IDXPTR approach to in-transaction indexing,
instead using an in-memory tdb to hold index values during a
transaction. This also cleans up a lot of the internal indexing logic,
hopefully making it easier to understand.

One of the big changes is in memory management, with a lot more use
made of talloc tricks to avoid copying dn lists, and shortcuts used to
avoid high intersection and union calculation costs.

The overall result is that a re-provision on my laptop goes from 48s
to a bit over 10s.

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