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.

Also, we probably should benchmark calling go exported methods from C++. If it's too slow, we may need to reconsider how we log from C++.