Fix spectator client not correctly reconnecting after shutdown

[peppy]

When switching servers over, I've notice a disparity in connections switching to the new spectator server (via datadog metrics). Users' clients would be stuck on the old spectator hub but also unable to make new spectator sessions.

Turns out this is due to incorrect use of SendAsync instead of InvokeAsync. The former does not block, nor propagate exceptions.

I'm not sure about the remaining SendAsync usage in SpectatorHub. We may want to switch them over as well.

I've tested this manually to work as expected.

[frenzibyte]

Multiplayer client already uses InvokeAsync on all methods, so I think it wouldn't hurt to change them here as well (would also allow propagating hub exceptions like multiplayer client does when calling one of their methods)

[peppy]

All the remaining usages of SendAsync were fire-and-forget, so seems best to update them, as that will have no effect on existing usages and is probably the more expected behaviour.