* client: allow reverting blocks past finality

* client: fix leaves reversion

* client: extend docs on revert

* client: add comment on leaves revert

Good night sweet prince

Previously one would create a sender and receiver channel pair, pass the
sender to the `build_network_future` through the service builder and
funnel network events returned from polling the network service into the
sender to be consumed by the authority discovery module owning the
receiver.

With recent changes it is now possible to register an `event_stream`
with the network service directly, thus one does not need to make the
detour through the `build_network_future`.

* client/authority-discovery: Limit number of connections to authorities

Instead of connecting to all sentry nodes of all authorities, with this
patch the authority discovery module does the following:

- Choose one sentry node per authority at random.

- Choose MAX_NUM_AUTHORITY_CONN out of the above at random.

The module uses randomness to prevent hot spots, e.g. all nodes trying
to connect to a single node. If the authority discovery module would
choose the nodes to connect to at random on each new address that it
learns of, the node would go through a lot of connection churn.  Instead
it creates a random seed at start up and uses this seed for its RNG on
each update cycle.

* client/authority-discovery: Extract address cache into own module

* client/authority-discovery/src/addr_cache: Add basic unit tests

* client/authority-discovery: Replace unwrap with expect on [u8] cmp

* .maintain/sentry-node/docker-compose.yml: Prefix endpoint flags

* client/authority-discovery/src/addr_cache: Use sort_unstable and cmp

* client/authority-discovery: Use BTreeMap in addr_cache for sorted iter

To reduce connection churn it is preferrable to have `get_subset` of the
`addr_cache` to return the same result on repeated calls. `get_subset`
iterates a map. To make the process of iteration deterministic, use a
`BTreeMap` instead of a `HashMap`.