We add the new direct failure detector to the list of services started
in the Scylla process.
To start the service, we need an implementation of `pinger` and `clock`.
`pinger` is implemented using existing GOSSIP_ECHO verb. The gossip echo
message requires the node's gossip generation number. We handle this by
embedding the pinger implementation inside `gossiper`, and making
`gossiper` update the generation number (cached inside the pinger class)
periodically.
`clock` is a simple implementation which uses `std::chrono::steady_clock`
and `seastar::sleep_until` underneath. Translating `steady_clock`
durations to `direct_failure_detector::clock` durations happens by taking
the number of ticks.
The service is currently not used, just initialized; no endpoints are
added and no listeners are registered yet, but the following commits
change that.
38f65e5a2e