Simplify and speed up the SeekOffsets code

[ankon]

• has: Use the similar function from the parent Map class, hoping that that
  is potentially faster than the brute-force approach through keys/some
• pop: Replace the complex and inefficient O(n)-ish logic with a simpler
  O(1) approach.
  We don't need to make an array of all entries, reverse that, and then pop
  one element -- we only need to know the first key/value pair.

[ankon]

Hold off on this one please.

has doesn't work this way unfortunately, because of how the Map equality works. The change in pop should still be fine.

[ankon]

Removed the has change; this should be good to go now.

I see the checks failing, and I have some issues locally on getting a stable test run -- looks a lot like flakes to me though (apart from the Azure database misconfiguration?)

[Nevon]

Could you merge latest master into this, and we can get it merged?

[ankon]

@Nevon sure :)