Increase default block size

- increase default size by 100K to 600K
- increase high-priority area from 27K to 30K
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
Silence useless warning in src/json/json_spirit_writer_template.h to make important warnings easier to see.

warning: typedef ‘Char_type’ locally defined but not used [-Wunused-local-typedefs]