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.

We really do need to move to have the Go engine interface expose snapshot functionality. We're going to need this to do an efficient job with MVCC scanning.