I've implemented fast-truncate support for VLCS.

Mostly it's possible to just discard pages; reuse of the discarded chunks of namespace gets directed to the append list of the next leaf page to the left in the tree.

The exception is that one must avoid discarding the leftmost child of a VLCS internal page, because the internal page structure prevents this namespace from shifting any further to the left. Instead one ends up with a gap between the internal page's start recno and the first child's start recno, and this leads to trouble.

Note that fast-truncate for FLCS has a whole additional set of problems because FLCS has to iterate through (rather than skip over) gaps in the namespace. Currently this doesn't look feasible to support.