Automatically initialize a ticker for each raft server
instance when `raft_services::add_server` is called.
A ticker is a timer which regularly calls `raft::server::tick`
in order to tick its raft protocol state machine.
Note that the timer should start after the server calls
its `start()` method, because otherwise it would crash
since fsm is not initialized yet.
Currently, the tick interval is hardcoded to be 100ms.
Tests: unit(dev, debug)
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>