sds: Implement periodic maintenance tasks for message synchronization

[adklempner]

1. Create dependency resolution system:

   • Scan incoming buffer for messages with missing dependencies
   • Track age of pending messages
   • Request missing messages from peers
   • Prioritize older pending messages
   • Handle timeout/failure cases for unresolvable dependencies

2. Implement unacknowledged message management:

   • Track age of unacknowledged outgoing messages
   • Implement priority-based rebroadcast strategy
   • Handle messages that exceed retry limits
   • Manage buffer size through pruning old messages
   • Track successful redelivery attempts

3. Add sync message system:

   • Track time since last message on each channel
   • Generate empty sync messages when channel is idle
   • Include current Bloom filter state in sync messages
   • Maintain minimum interval between sync messages
   • Handle sync message priority vs regular messages

4. Create maintenance scheduler:

   • Implement configurable maintenance intervals
   • Coordinate different maintenance tasks
   • Handle task failures gracefully
   • Provide monitoring/metrics for maintenance operations
   • Support manual trigger of maintenance tasks