unexpected blocking in call to `MonitorEvents` (Docker 1.7)

[tbg]

it appears as of recently (highly likely since Docker 1.7) the behavior of MonitorEvents has changed: it'll block until the first event is received. While it's blocking, it's stuck on the http client's Get. This is unexpected since we expect to get a channel back right away, and may only then do work that prompts further events.

See https://github.com/cockroachdb/cockroach/pull/1647