Remove deepcopy of client._http

[liyanhui1228]

Fixes #3931.
When GOOGLE_CLOUD_DISABLE_GRPC=true, the logging will have 'AuthorizedSession' object has no attribute 'credentials' exception.
The line http = copy.deepcopy(client._http) is a relic when httplib2 was used. Removing it and directly use the client passed in instead of re-initialize with the deepcopied http object solved the problem.

[lukesneeringer]

Side note: CODEOWNERS does not seem to work quite right. :-(

[lukesneeringer]

On the merits of this actual PR -- this does look sane to me, but hoping @tseaver or @jonparrott will make sure I am not missing something obvious.

[waprin]

The original deepcopy was a race condition for the async long handler, but it's covered in the system test, being a race condition it didn't always appear but it did frequently enough that if there's any problem with the PR you'd probably know soon. LGTM if jon think it look right.

[liyanhui1228]

Thanks @waprin! I have run the system tests for several times locally and it works fine. Though the race condition does not always appear as Bill said. If thread-safety is the only reason for deepcopy, sounds like we can safely remove it. Can we go ahead and merge this?

[liyanhui1228]

The system test test_create_sink_pubsub_topic failed with error message from google.cloud.pubsub import client as pubsub_client ImportError: cannot import name client, which seems need to be updated to match the recent pubsub changes. Probably fix it in another PR?

[lukesneeringer]

@liyanhui1228 Go ahead and fix that here. Happy to merge once tests pass.

[liyanhui1228]

Ready for merging, system tests passed locally.

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