intervals: add support to automerge with kernel elements

intervals: add support to automerge with kernel elements

Extend the interval codebase to support for merging elements in the
kernel with userspace element updates.

Add a list of elements to be purged to cmd and set objects. These
elements representing outdated intervals are deleted before adding the
updated ranges.

This routine splices the list of userspace and kernel elements, then it
mergesorts to identify overlapping and contiguous ranges. This splice
operation is undone so the set userspace cache remains consistent.

Incrementally update the elements in the cache, this allows to remove
dd44081d91ce ("segtree: Fix add and delete of element in same batch").

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>