indexes: Don't wipe indexes again when continuing a prior reindex

When restarting bitcoind during an ongoing reindex without setting the -reindex flag again, the block and coins db is left intact, but any data from the optional indexes is discarded. While not a bug per se, wiping the data again is wasteful, both in terms of having to write it again, as well as potentially leading to longer startup times. So keep the index data instead when continuing a prior reindex.

Also includes a bugfix and smaller code cleanups around the reindexing code. The bug was introduced in b47bd959: "kernel: De-globalize fReindex".