fix: shed: backfill events from correct starting height

[arajasek]

Related Issues

If we want to get events and other indices for the from height, we need to be starting from a tipset that is guaranteed to be later than from. The current code will incorrectly return the tipset at from in the event that the tipset at from+1 is null, and thus miss one epoch.

Proposed Changes

Request the tipset using ChainGetTipSetAfterHeight(from+1). If from+1 is non-null, this will make no difference. If from is null, this will now return the first non-null tipset at a greater height, thus giving us a tipset whose parent is at from height (or earlier, assuming from itself is null). I believe that's what we want for this code.

Additional Info

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