Merge #12401: Reset pblocktree before deleting LevelDB file

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

Author: laanwj

src/init.cpp

@@ -1425,6 +1425,9 @@ bool AppInitMain()
                 pcoinsTip.reset();
                 pcoinsdbview.reset();
                 pcoinscatcher.reset();
+                // new CBlockTreeDB tries to delete the existing file, which
+                // fails if it's still open from the previous loop. Close it first:
+                pblocktree.reset();
                 pblocktree.reset(new CBlockTreeDB(nBlockTreeDBCache, false, fReset));
 
                 if (fReset) {