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 leveldb to 1.13

This fixes some further MacOS corruption issues.

Merge commit 'c3d342d5bd5cbb2bebcfd01f2dd312088fb3aa79' into 0.8.6
Don't waste work on slow peers until they catch up