initial support for transactions
Change RevTree buckets to contain the new Bucket class  and optimizations for bbox filtering.

Non leaf RevTrees used to contain a set of ObjectId pointing to the bucket trees.
A class Bucket has been introduced, sharing the common Bounded superinterface with Node.
Tree traversals can now receive a bounding box filter and skip trees, buckets, and feature nodes
that do not satisfy the filter without having to parse the features or travers unnecessary trees.
tmp commit, missing support for empty trees in commits needed for createSchema?
createSchema working with a transaction