Potential bug in ledger fetcher

### Subject of the issue

Exiting at https://github.com/algorand/go-algorand/blob/master/catchup/ledgerFetcher.go#L178 is wrong because `tarReader` might still contain buffered data. The current implementation of `tar.Reader` doesn't use buffering, but nothing in its contract says it doesn't. Particularly, its implementation might change in the future and it will become an issue.

The best solution, imo, is to not return anything in `util.WatchdogStreamReader.Reset()` since it's not necessary.