- added an authority struct
  - consists of controller (the xds stream) and an pubsub (cache for resources received from this authority)

- an authority is created when the first watch is called on this authority, and deleted when the last watch is canceled
  - authorities are ref-counted
  - authorities are kept in a `map[server-config]authority`, note that key is the config, not authority name, so that two authoritie with different names but same config will share the xds stream and pubsub
  - authority is kept in a cache after deletion, and only actually deleted after a timeout, so it can be revived if a new watch is started

- watch functions find (or build) the authority first, then call watch on this authority
  - they also handle ref/unref of the authority

- dump (for CSDS) is updated to dump resources from all the authorities

- resource handling (when unmarshalling receive xds resources) parses the resource name first, and then turns it back to a string, to canonicalize (especially the context parameters)

- tests
  - update handler is only available after the watch is started, so many tests need updating
  - make many LDS/RDS/CDS/EDS tests share code, and also added a federation version