Redis connection failures have way too much impact

[bdach]

@ThePooN discovered during some infra work that a Redis connection failure will bring down spectator flows completely with

As is, redis definitely wasn't supposed to be this service-critical as its only purpose was delivering user statistics updates which should not be bringing down the entirety of spectator flows. It was definitely not intended, and is only happening due to DI behaviour foibles combined with signalr design (tl;dr: hubs are instantiated per client RPC call, which means that a service failing to resolve - such as IConnectionMultiplexer - will cause all of the operations that touch that hub to fail).

We probably need to put some kind of backstop against that happening in place. Or possibly even reconsider osu-server-spectator middlemanning in this process whatsoever and take a look at something like https://centrifugal.dev/ instead.