Diag. Events: Add local persistency

Some generated events will be rather less frequent but very useful for retroactive troubleshooting. E.g. all events related to bootstraping and gossip would probably be worth saving, as they might provide valuable insights and will consume very little resources in low quantities. Imaging if we could e.g. in case of CASSANDRA-13348 just ask the user to run a tool like ./bin/diagdump BootstrapEvent on each host, to get us a detailed log of all relevant events provide a dump of all events as described in the documentation.

This could be done by saving events white-listed in cassandra.yaml to a local table. Maybe using a TTL.