The srml/authority-discovery module implements the OneSessionHandler in
order to keep its authority set in sync. This commit adds the module to
the set of session handlers.

Instead of network worker implement the Future trait, have it implement
the Stream interface returning Dht events.

For now these events are ignored in build_network_future but will be
used by the core/authority-discovery module in subsequent commits.

When the channel from network service to network worker is finished,
also finish the network worker stream given that both network worker and
network service have the same lifetime.

Authority discovery implements the future trait. Polling this future
always returns NotReady. The future uses Tokios timer crate to become
proactive periodically, e.g. advertising its own external addresses.

This commit ensures that the underlying Tokio task is always registered
at the Tokio Reactor to be woken up on the next interval tick. This is
achieved by making sure `interval.poll` returns `NotReady` at least
once within each `AuthorityDiscovery.poll` execution.

Kademlia's default time-to-live for Dht records is 36h, republishing
records every 24h. Given that a node  could restart at any point in
time, one can not depend on the republishing process, thus starting to
publish  own external addresses should happen on an interval < 36h.

In addition have the first tick of the interval be at the beginning not
after an interval duration.

Abstract NetworkService via NetworkProvider trait to be able to mock it
within the unit tests. In addition add basic unit test for
`publish_own_ext_addresses`, `request_addresses_of_others` and
`handle_dht_events`.

Split the global `AuthorityDiscovery.interval` into two intervals:
`publish_interval` and `query_interval`. Dht entries of other
authorities can change at any point in time. Thereby one should query
more often than publish.

The authority discovery module treats an authority identifier as an
opaque string. Thus the type abstraction `AuthorityId` is unnecessary,
bloating the `core/service` construction code.

…very

Instead of instantiating the authority discovery module within
core/service, this commit instantiates authority discovery within
node/cli. The authority discovery module depends on the srml authority
discovery module, which depends on the im online module, as well as
session, ...

With the former approach all these dependencies were enforced on any
substrate implementation. With the latter approach these dependencies
are optional.

…very

…very