BUG-8665: fix memory leak around RangeSets

This is a thinko on my part, where I was thinking in terms of a
discrete set (UnsignedLong) and assumed RangeSets will coalesce
individual items.

Unfortunately TreeRangeSet has no way of knowing that that the
domain it operates on is discrete and hence will not merge invididual
ranges.

This patch fixes the problem by using [N,N+1) ranges to address
the problem. A follow-up patch should address this in a more
efficient manner.

Change-Id: Iecc313e09ae0cdd51a42f7d39281f7634f0358a7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>