Fixed PruneOutdatedChains()

[greymistcube]

Notes:

• The new implementation works only under an assumption that the Stack of chain infos are either sequential or overlapping.
  • This is much weaker assumption than the original where every chain info's index has been managed correctly and is uncorrupted.
• Some implementation decisions are made assuming that this will be a one-off "script" to prune outdated chains.
  • Internally, int indices are used pretty haphazardly assuming that currently there is no chain with its Tip.Index exceeding the max int value.
  • Again, assuming that there is no chain that is "too long", internal iteration instantiates an entire list of BlockHashes, which should not be a problem in terms of memory usage.
  • Due to using List<BlockHash> for iteration, the method should be safe (will throw an Exception when it fails without writing to IStore).