Ensure that any bodies created in GetBody are cleaned up

[kevinconaway]

This PR fixes issues brought up in:

googleapis/google-cloud-go#1651
googleapis/google-cloud-go#1380

As best I understand it, the goroutines will leak when GetBody is called and the resulting body is not closed.

This PR modifies UploadRequest to add any request bodies created by GetBody to a list to be cleaned up by the cleanup function that is returned. Previously, only the main request would be cleaned up but not any of those returned by GetBody.

I've verified that this fixes the issue in our environment.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[kevinconaway]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[jeanbza]

Hello! Thank you for investigating this.

@cjcotter @frankyn - the biggest client that uses this is storage AFAIK. Could one of you investigate this with fairly high priority? This may fix some of the other subtle issues we've been seeing.

@kevinconaway Unfortunately this github repo is just a mirror of another git repo that uses gerrit for code review. Would you mind closing this PR and submitting a change request to gerrit? Our CONTRIBUTING.md has extensive guidance, and I'm happy to help too. If you prefer not, I can submit this for you, but unfortunately then you wouldn't get credit which is not as cool.

[kevinconaway]

Thanks, I'll do so.

[kevinconaway]

Created a gerrit CL here https://code-review.googlesource.com/c/google-api-go-client/+/48910