It is possible that a span update happens during a milestone.
A headers slice might cross to the new span.
Also if 2 forks evolve simulaneously, a shorter fork can still be in the
previous span.
In these cases we need access to the previous span to calculate
difficulty and validate header times.
SpansCache will keep recent spans.
The cache will be updated on new span events from the heimdall.
The cache is pruned on new milestone events and in practice no more than
2 spans are kept.
The header difficulty calculation and time validation depends on having
a span for that header in the cache.
