Implementing backward compatibility for engine-api users about engine-api HOT 3 OPEN

cc @calavera @dongluochen

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I think the only reasonable way we've found to do this is by checking the version in the http request. I'm super open to suggestions, but I don't think there is any easy way to handle those. See what we do for docker inspect:

https://github.com/docker/docker/blob/master/daemon/inspect.go#L19-L27

Re p1v18: there was nothing defined to keep backward compatibility around before p1v19 😭

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Thanks @calavera.

For reference, here's how Engine does it by defining a separate function: https://github.com/docker/docker/blob/2658341b5fe9dec5c4839f0721219111fb08b43f/daemon/inspect_unix.go#L24

And then using a wrapper function: https://github.com/docker/docker/blob/2658341b5fe9dec5c4839f0721219111fb08b43f/daemon/inspect.go#L19

(Thanks to @tiborvass for pointing that out).

Also, a related issue is this, requiring to always specify API version: moby/moby#21157

(cc @thaJeztah)

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