Leveldb 1.7

This is a tweaked version of pull request #2106 that will cross-compile on Lucid, so it is (I hope!) compatible with the pull-tester and our old gitian-building environment.

 * Upgrade to LevelDB 1.7
 * Switch to a native Windows LevelDB port (not anymore via Boost)

I've been testing this versus the boost-based, old-leveldb code in git HEAD.