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.
Fleshed out the documentation in db.h.
Address code review comments by adding more comments.
Merge pull request #128 from cockroachdb/pmattis/roachlib

Provide our own C interface to rocksdb.