Add more checks for rewinds for subgraphs that have ongoing pruning enabled

[azf20]

When rewinding a subgraph, Graph Node should ensure that data for the specified block hasn't been pruned

[lutter]

A rewind should not be allowed to go back beyond earliest block + reorg threshold - we need at least these many blocks; we could raise that a little more to have a safety threshold, but this is the absolute minimum.