uSync.Complete: v17.4.0 and v18.1.1 releases

Updated CHANGELOG.md for the v17.4.0 release (restore point live progress, dedicated background queue, uSync 17.3.8 / Jumoo.Processing 17.4.0 bump)
Published the v17.4.0 release notes on the public issue tracker, covering everything since v17.3.6 (17.3.9-17.3.11 patch releases plus the new work)
Bumped uSync to 18.1.2 and Jumoo.Processing to 18.1.1 on v18/main: wired the new async uSyncCallbacks delegates in Publisher's SignalR progress hooks, and threaded a CancellationToken through CreateRestorePoint into the pipeline call
Published the v18.1.1 release notes on the public issue tracker, and tagged v18.1.1 in the repo