Provide our own C interface to rocksdb.

Stop using the rocksdb C interface and instead provide our own C
interface to rocksdb. This makes it cleaner to implement various bits of
cockroach-specific functionality in C++ such as the compaction filter
logic and merge operator.

Heh, guess I didn't beat that horse long enough. Yeah, happy to switch over to the viewfinder model in a subsequent change.

Do you think DBSnapshot should be a type of DBEngine as well?