This is motivated by errors such as
https://sentry.gitlab.net/gitlab/gitlabcom/issues/1766076, where
attempts to move nulls fail even with rebalancing due to the fact that
the gap found is not big enough for all the nulls.

We address this in two ways:

- we try to consolidate multiple gaps. Up to 10 gaps will be found and
consolidated to make enough room. This is expected to be effective in
most cases.

- if we still cannot make enough space, rather than failing, we stack
all overflow elements at the edge. So if we need to move 10 nulls to the
end, and we only have space for 5, all the extras (5 of them) will be
placed at the maximum position.

Adds a changelog entry since this is a bug-fix