Add tombstone GC for GossipListValue

[noha]

Tombstones (removed items) were never physically dropped from the entries dict, only ever marked present: false — fine for correctness (each GossipListValue is always replaced as a whole atomic unit via the enclosing GossipApplicationState's version, so there's no per-item merge/resurrection risk the way Cassandra's cell-level tombstones have), but unbounded over a long-running process with many distinct items added then removed.

gcTombstonesOlderThan: drops any tombstoned entry whose itemVersion is more than the given gap behind the list's own versionCounter. Uses the list's own logical version clock rather than wall-clock time, since nothing in Gossip carries real timestamps. Deliberately left as an explicit method the caller invokes with its own threshold, not auto-wired into Gossiper's periodic gossip loop — picking a concrete grace window is a policy call that depends on gossip round frequency, not something to default silently.

New GossipListValueTest (5 tests) covers add/remove/tombstone semantics and the GC boundary (kept when recent, removed when past the gap, present items never touched).