lock around read operations in graph

Writes and reads will fail with ErrBusy if there's
concurrent reads or writes, respectively. It is
not sufficient to only lock around writes.
gograph: Use RWMutex to allow concurrent readers
gograph: allow Walk() reentrance

Hold the read lock while reading the child graph,
then walk over the children without any lock, in
order to avoid deadlock.
testing infrastructure, issue #1800: Refactor docker testing using Docker in Docker
docker-ci 0.35. Push docker coverage and testing into docker-ci production. Update docker nightlyrelease
docker-ci 0.36. Patch hack/dind with latest code for nightly release to work.
docker-ci 0.37. Patch hack/dind with latest code for docker-test.
docker-ci 0.40. Migrate docker-ci to Digital Ocean.
docker-ci 0.4.5: Sync tests with progress in docker and docker-registry. Use revamped shiny DinD.
fix mergeConfig with new ports
Deprecate old tagging format
prevent panic if you use API in a wrong way
Merge pull request #2584 from dotcloud/docker-ci

docker-ci production sync
Merge pull request #2599 from dotcloud/prevent_panic_wrong_api_use

Prevent panic if you use API in a wrong way
Merge pull request #2597 from dotcloud/deprecate_separate_tag

Deprecate old tagging format
Merge pull request #2593 from dotcloud/2578

Fix mergeConfig with new ports
Merge pull request #2565 from danielnorberg/master

lock around read operations in graph