Squashed 'src/leveldb/' changes from be1b0ff..be91006

936b461 Merge upstream LevelDB 1.13.
748539c LevelDB 1.13

git-subtree-dir: src/leveldb
git-subtree-split: be9100673b05cec1662a54d0b7a59e4317fdda86
Upgrade to Bitcoin LevelDB 1.13

Merge commit '2a6cdcf6e57922fec0219b5966df85c8417d6ba3' into master-0.8