Reset setBlockIndexCandidates once block index db loaded

Delete all entries in setBlockIndexCandidates that are worse than our new current block upon completion of the initial LoadBlockIndexDB.

Please check for end before dereferencing *it:
```
    while (it != setBlockIndexCandidates.end() && setBlockIndexCandidates.value_comp()(*it, chainActive.Tip())) {
```