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.

Actually, this is necessary or else txnPrefix will never match the prefix of an actual key. The previous code was effectively doing `strlen(txnPrefix.data)`.