pd: use a range query for client synchronization

[erwanor]

This PR:

• implements a stream_compact_block method to compact_block::StateReadExt
• remove the block_fetcher tokio tasks in ObliviousQueryService::compact_block_range
• use a single range query to catch-up on existing CompactBlocks

[erwanor]

Merging optimistically since the range query functionality is relatively well tested. The main unknown is whether the anticipated performance improvement will materialize or not. If it doesn't this PR will be easy to revert. This closes the loop on a series of pd improvements as a response to #2867.