uSync: async callback surface on uSyncCallbacks, v17.3.8 release

Repo: KevinJump/uSync

Merged #1049: added a non-breaking async callback surface to uSyncCallbacks (CallbackAsync, UpdateAsync, SetRangeAsync, IncrementalUpdateAsync, CompleteAsync + Raise*Async helpers) so consumers like uSync.Complete's SignalR hub clients can await a send instead of blocking with .Wait()
v17/main is now the default/LTS branch — STS branches (v18/main etc.) are follow-on, merged forward from it
Released v17.3.8 to NuGet