* Fix syncing bugs by recursively attempting to process parents in the import queue, change BlockRootsIterator
* Swap from crossbeam channel to tokio mpsc
* Recursion fix
* Remove exess block processing
* Fix network lag, correct attestation topic
* Correct network poll logic
* Overhaul of SimpleSync and modify BlockRootsIterator to return start_slot
* Fix bug in tests relating to StateRootsIterator
* Remove old, commented-out heartbeat code.
* Tidy docs on import queue enum
* Change source logging msg in simple sync
* Rename function parameter in simple sync
* Use `BestBlockRootsIterator` in `reduced_tree`
* Update comments for `BestBlockRootsIterator`
* Fix duplicate dep in cargo.toml
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