index: write out the tree cache extension

Keeping the cache around after read-tree is only one part of the
optimisation opportunities. In order to share the cache between program
instances, we need to write the TREE extension to the index.

Do so, taking the opportunity to rename 'entries' to 'entry_count' to
match the name given in the format description. The included test is
rather trivial, but works as a sanity check.
index: make sure to write cached subtrees if parent is invalidated

If e.g. the root tree is invalidated, we still want to write out
its children, since those may still have valid cache entries.
index: fill the tree cache on write-tree

An obvious place to fill the tree cache is on write-tree, as we're
guaranteed to be able to fill in the whole tree cache.

The way this commit does this is not the most efficient, as we read the
root tree from the odb instead of filling in the cache as we go along,
but it fills the cache such that successive operations (and persisting
the index to disk) will be able to take advantage of the cache, and it
reuses the code we already have for filling the cache.

Filling in the cache as we create the trees would require some
reallocation of the children vector, which is currently not possible
with out pool implementation. A different data structure would likely
allow us to perform this operation at a later date.
Update CHANGELOG and PROJECTS with the tree cache changes