CollectTipSetOfHeightAtLeast new OBO error

[ZenGround0]

Description

Because of the recent fix in #3026 we are now missing one tipset in the ancestor subchain in the case when the sample height falls on a null block. This bug should only be an issue in the case a PoSt is submitted at the very last possible block. In that case validators will incorrectly fail with "sample height out of range"

Acceptance criteria

• Modify CollectTipSetsOfHeightAtLeast to handle this OBO error
• Simplify the GetRecentAncestors code which has some complexity that will go away when making this change. Specifically we no longer need two iterators because CollectTipSetsOfHeightAtLeast will no longer drop a value needed by CollectAtMostNTipSets
• Test this edge case

Risks + pitfalls

Where to begin

[anorth]

See also #3025

[ZenGround0]

@deaswang to clarify a bit to help you with #3250 the major fix that this issue is tracking is to make CollectTipSetsOfHeightAtLeast return all tipsets up to and including the first tipset with a height less than or equal to the input height. The reason is that this change in PR #3026 modified the sampling behavior during validation to expect all of these tipsets but did not modify the sampling behavior during tipset collection.

tl;dr CollectTipSetsOfHeightAtLeast should traverse up to minHeight (as it does now) and additionally include the first tipset less than minHeight if there is no tipset at minHeight.