Add a circuit breaker for validating access tokens

config.sample.yaml

...hub.com/turt2live/matrix-media-repo/client/r0/download.go

....com/turt2live/matrix-media-repo/client/r0/preview_url.go

...ithub.com/turt2live/matrix-media-repo/client/r0/upload.go

src/github.com/turt2live/matrix-media-repo/config/config.go

src/github.com/turt2live/matrix-media-repo/matrix/matrix.go

src/github.com/turt2live/matrix-media-repo/util/matrix.go

vendor/manifest