uSync: SignalR skip negotiation support

Repo: KevinJump/uSync

v17: SignalR client now respects Umbraco's ClientShouldSkipNegotiation setting (Umbraco 17.5.0+), connecting over WebSockets without the negotiate call that 405'd (#1169, fixes #1170)
v17: added a GetConnectionId hub method so the client still gets its connection id when negotiation is skipped, so per-item progress keeps flowing
v17: SignalR context stops the old connection before reconnecting and catches start() failures
Forward-ported to v18 (#1171) and v19 (#1172); v19 keeps its cookie-only auth
Closed #1152 (v19 legacy-flow migrators), done in #1155