Temporarily revert to writing .sst files instead of .ldb.

See discussion in bitcoin/bitcoin#3529.

This reverts to writing .sst files (the only option in LevelDB 1.13 and below) instead of .ldb files (the default in 1.14 and above). 1.14 and above do read both .sst and .ldb though, so this makes the code backand and forward comaptible with every other version.