Use RPC providers to speed up syncing from old snapshots

# Summary

Syncing from an old snapshot requires downloading a lot of tipsets (2880 per day the snapshot is behind). We download the tipsets from the libp2p swarm in batches of 16, and we're limited to a single batch at a time. This sequential download is often slower than the validation of the tipsets, and validation cannot begin until all the tipsets have been downloaded.

RPC providers offer a `ChainGetTipSetByHeight` method that returns a tipset for a given height. We can use this method as an alternate, non-libp2p source of tipsets. Instead of sequentially fetching a single range of tipsets from the swarm, we can use RPC providers to split the range into smaller ranges fetched in parallel.

# Requirements

Fetching headers starting from a 1-week-old snapshot should at least be 10x faster.

# Motivation

I want syncing from older snapshots to be faster.