Cluster: Prune association tombstones

[ktoso]

Associations get tombstoned when we cut connections to them.

We must cleanup the tombstones every settings.cluster.associationTombstoneTimeToLive time.
That time is normally set something "very large" so days or many hours. Default now is one day, could be perhaps 12h but let's go with a day for a start I guess (other systems I know use many days, though they're less dynamic than our clusters).

    /// Defines when an association shall be completely dropped from the tombstones list.
    /// An association ("unique connection identifier between two nodes") is kept as tombstone when severing a connection between nodes,
    /// in order to avoid accidental re-connections to given node. Once a node has been downed, removed, and disassociated, it MUST NOT be
    // communicated with again. Tombstones are used to ensure this, even if the downed ("zombie") node, attempts to reconnect.
    public var associationTombstoneTimeToLive: TimeAmount = .hours(24) * 1

How to solve

Schedule a periodic (every 10 minutes or so) cleanup tick in ClusterShell and have it skim over the tombstones and other cleanups it may need to do.

[ktoso]

TODO: Check if SWIM needs similar or not