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.

Eh? rocksdb::WriteBatch copies the passed in keys and values (I just double-checked this behavior). So does leveldb::WriteBatch. We kept our own separate key/value map for batches in viewfinder because WriteBatch does not provide adequate support for iteration.