a8b5d20 Reset pblocktree before deleting LevelDB file (Sjors Provoost)
Pull request description:
#11043 repaced:
```
delete pblocktree;
pblocktree = new CBlockTreeDB(nBlockTreeDBCache, false, fReset);
```
With:
```
pblocktree.reset(new CBlockTreeDB(nBlockTreeDBCache, false, fReset));
```
This is problematic because `new CBlockTreeDB` tries to delete the existing file, which will fail with `LOCK: already held by process` if it's still open. That's the case for QT.
When QT finds a problem with the index it will ask the user if they want to reindex. At that point it has already opened `blocks/index`. It then runs this [while loop](https://github.com/bitcoin/bitcoin/blob/v0.16.0rc3/src/init.cpp#L1415) again with `fReset = 1`, resulting in the above error.
This change makes that error go away, presumably because `reset()` without an argument closes the file.
Tree-SHA512: fde8b546912f6773ac64da8476673cc270b125aa2d909212391d1a2001b35c8260a8772126b99dfd76b39faaa286feb7c43239185fe584bd4dc2bc04a64044ce
Backport of Core PR12401