Release leveldb 1.10

Fixes issues
147 - thanks feniksgordonfreeman
153
156
166

Additionally,
* Remove calls to exit(1).
* Fix unused-variable warnings from clang.
* Fix possible overflow error related to num_restart value >= (2^32/4).
* Add leveldbutil to .gitignore.
* Add better log messages when Write is stalled on a compaction.