[ASNodeController+Beta] Provide an option to allow nodes to own their controllers.

[appleguy]

We should certainly remove this before moving ASNodeController out of Beta. However, I think
it will require at least ASCollectionNode to be able to retain its top level set of node controllers.

Without this facility built in, it's very difficult for apps supporting both UIKit and ASDK to
manually manage the controllers and keep them in sync with perfect timing.

[garrettmoon]

Sorry I'm bad at shell scripts so the build failure didn't get reported:

 -> Texture (2.3)
    - WARN  | [Texture/Core, Texture/PINRemoteImage, Texture/IGListKit, and more...] xcodebuild:  /usr/local/var/buildkite-agent/builds/BuildKites-Mac.local-1/pinterest/texture/Source/ASNodeController+Beta.m:49:13: warning: assigning retained object to weak property; object will be released after assignment [-Warc-unsafe-retained-assign]
 
[!] Texture did not pass validation, due to 1 warning (but you can use `--allow-warnings` to ignore it).
[!] The validator for Swift projects uses Swift 3.0 by default, if you are using a different version of swift you can use a `.swift-version` file to set the version for your Pod. For example to use Swift 2.3, run: 
    `echo "2.3" > .swift-version`.
You can use the `--no-clean` option to inspect any issue.

[ghost]

🚫 CI failed with log

[ghost]

	1 Warning
⚠️	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

Generated by 🚫 Danger

[appleguy]

This is ready for review when folks have a chance. Not at all urgent.

It would be great to not need this diff if changes are planned soon to make it unnecessary. However, I think it's perfectly fine if this is not on the priority list for the core team - thus this diff so that there's a good option for clients to work around in the meantime.