service/src/lib: Update authority discovery construction (#1563)

* service/src/lib: Update authority discovery construction

https://github.com/paritytech/substrate/pull/6760 introduces the concept
of an authority discovery `Service` allowing one to communicate with an
authority discovery `Worker`, e.g. to learn the `Multiaddr`s for a given
`AuthorityId`.

Along with the new `Service` structure it also alters the authority
discovery constructor to return both a worker and a service. This
commit adjusts the callside of the constructor, ignoring the `Service`
for now.

* "Update Substrate"

* Revert ""Update Substrate""

This reverts commit 04fb79c4.

* Update Substrate

Co-authored-by: parity-processbot <>
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