lighthorizon: Extend the indexing tool to allow unbounded ledger ranges

[Shaptic]

The ledgerexporter tool will dump ledgers into txmeta forever, so the indexer should be aware of this.

This might involve something like fsnotify watching the -source parameter of the indexer. However, that's is only appropriate for local filesystems, so another solution is needed for remote ledger dumps.

Whether or not polling is acceptable is an open question.