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.

TODO: I still need to document the various bits in roachlib/db.{cc,h}. Just wanted to get this out there so that it stops rotting on my laptop.