Sync: PSR-4 the sync server

[tyxla]

In #12712 and all the related PRs we are making an effort to namespace all the sync modules. We also need to start namespacing the rest of the Sync core, see #12751.

This PR adds a namespace to the sync server and that way autoload it with PSR-4.

Changes proposed in this Pull Request:

• Sync: Namespace the sync server and autoload it with PSR-4.

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

Part of the Jetpack DNA project - p1HpG7-70O-p2

Testing instructions:

• Checkout the branch.
• Enable WP_DEBUG_LOG.
• Trigger a full sync.
• Verify sync works well and you have no errors logged.
• Verify tests pass and CI is green.

Proposed changelog entry for your changes:

• Sync: Namespace the sync server and load it with PSR-4

[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

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[lezama]

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