Convert Jetpack_XMLRPC_Server to a classmapped package.

[zinigor]

This replaces #12568

Changes proposed in this Pull Request:

• This is part of the DNA project. This PR moves the XMLRPC server class to a package of its own.

Testing instructions:

• Apply this PR, make sure you can connect/disconnect Jetpack successfully.

Proposed changelog entry for your changes:

• N/A

[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: July 2, 2019.
Scheduled code freeze: June 25, 2019

Generated by 🚫 dangerJS against fda3dd1

[zinigor]

Not merging or moving ahead with this now because #12568 (comment). The direction we take in #12699 will help us figure out if we need to classmap this class.