Previously, we would store authz providers globally and refresh them
every now and then.
However, creating the providers is fairly cheap (1.3ms in a local trace)
so we should not keep them in memory and remember to not forget to start
the watcher routine.
This will help for multi-tenant Sourcegraph in that providers are now
computed for the context in question, and not held globally. Keeping
potentially 100k authz providers in memory will not scale.
Test plan: Still works, local Jaeger traces are quite acceptable.