[ECO-5461] Implement tombstoning and garbage collection spec

[lawrence-forooghian]

Note: This PR is based on top of #45; please review that one first.

Implements ably/specification#350. Have not yet done unit tests; have deferred those to #52, and will instead prioritise porting across the JS integration tests.

Summary by CodeRabbit

• New Features

  • Added automatic cleanup (garbage collection) of deleted (tombstoned) objects and map entries after a configurable grace period.
  • Objects and map entries now track when they were deleted, improving state management and consistency.
  • Introduced explicit tombstone state and timestamp properties for live counters, maps, and entries.
  • Added support for tombstone-aware operations including object deletion and entry removal.
  • Enabled periodic background garbage collection of tombstoned objects in real-time objects.

• Bug Fixes

  • Improved handling of deleted objects to prevent unintended operations or data access.

• Tests

  • Updated tests to reflect new deletion tracking and cleanup logic.

• Chores

  • Enhanced internal logging for deletion and cleanup events.