Explicitly specify which golang tag we want.

Not specifying a tag causes all of the golang containers to be
downloaded even though only golang:latest will be used. This is
excessive in terms of time and disk space.
Merge pull request #131 from cockroachdb/pmattis/docker

Explicitly specify which golang tag we want.
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.