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.

Yes. Same as with Viewfinder. It'll simplify the go code as well--so annoying to have to call SnapshotGet(), etc. And having to create a batch object and then commit it when you don't really need the batch functionality is a pain.