Implement the Clients engine and integrate it with the sync manager

[thomcc]

The clients engine has to be tightly integrated with the sync manager, and is required for the sync manager to function properly.

This has to be done after #1379 (although in practice "after" might be a bit relative, since this will probably be done in a rolling PR a la history, bookmarks, etc), but can should be able to be be done mostly concurrently with the other parts of the sync manager.

Also part of the sync manager work (#1217)

┆Issue is synchronized with this Jira Story
┆Story Points: 8
┆Sprint: SYNC - 2019-09-26
┆Epic: [Q3 1.1] Sync Manager / CWTS