raft: replication test: tickers on raft_cluster
raft: replication test: tickers on raft_cluster

Move tickers to raft_cluster helper class. Ticker initialization and
pause is done automatically at start_all() and stop_all().

Add temporary helpers to manage specific tickers. These might be removed
later once proper node abort and reset are implemented.

Signed-off-by: Alejo Sanchez <004be06a28e38b437b038f9b5c0b9f5758b792e9@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla