Connection: Implement disconnect_user in the connection package

[tyxla]

This PR implements the disconnect_user in the connection package. This method was previously implemented in the Jetpack class and wasn't yet implemented in the connection package.

We also update old method usage to use the new connection manager method, and deprecate the old method in favor of the new one.

Changes proposed in this Pull Request:

• Connection: Update docs of Manager_Interface::disconnect_user
• Connection: Implement Manager::disconnect_user()
• Connection: Deprecate Jetpack::unlink_user in favor of the connection package
• Connection: Add todos to refactor XMLRPC client loading
• Connection: Use Manager::disconnect_user in Jetpack class
• Connection: Use Manager::disconnect_user in Jetpack CLI
• Connection: Use Manager::disconnect_user in core REST API endpoints
• Connection: Use Manager::disconnect_user in XMLRPC server

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This is a continuation of this effort: p1HpG7-7lI-p2

Testing instructions:

• A secondary user in this context is a user that is connected to WP.com but is not the master user / connection owner.
• Disconnect a secondary user from WP.com and make sure it goes correctly without any errors.
• Disconnect a secondary user from WP admin and make sure it goes correctly without any errors.
• Disconnect a secondary user using WP CLI and make sure it goes correctly without any errors.
• Make sure all tests still pass.

Proposed changelog entry for your changes:

• Connection: Implement Manager::disconnect_user()
• Connection: Deprecate Jetpack::unlink_user in favor of the connection package
• Connection: Use Manager::disconnect_user instead of Jetpack::unlink_user everywhere.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: September 3, 2019.
Scheduled code freeze: August 27, 2019

Generated by 🚫 dangerJS against c004420

[lezama]

nice! I like disconnect_user much more.

🚢 IT!