Heimdall prepares the next span a number of sprints before the current
span ends. Currently we always fetch the next span regardless of which
sprint we are in during the current span. This causes a liveness issue
due to how the Heimdall client works (it infinitely retries until it
fetches a span - this issue will be fixed in a separate PR). This PR
fixes this by matching what bor does - it fetches the next span only in
the last sprint of the current span.
Changes:
- Adds a unit test for the above
- Adds a new function BlockInLastSprintOfSpan
- Some code reorg and cleanup - moves the span num related functions
from the bor package to the span sub package for better logical grouping
