sentry: Replace policy watcher with API command

[abukosek]

Currently, the sentry gRPC worker (worker/sentry/grpc/worker.go) watches for policy changes, but that is unnecessary, since the upstream node can simply push the changes to the sentry using a new sentry API command (e.g. UpdatePolicies) in a similar way as was recently done with SetUpstreamTLSCertificates in PR #2675.

This would also enable simplification of the sentry gRPC worker code (e.g. not storing the connection dialed in initConnection() in the worker struct, removing other cruft, etc.).