Decouple block queue pruning from block announcements

[ii-cruz]

What's in this pull request?

The live sync has an invariant that all blocks come with no gaps from the gossip sub.
Before #2982 we had a bug where peers (active validators) were being miscategorized as not synced. Thus we would ignore their blocks from the gossip sub, creating gaps during the syncing process. Every gap would generate a missing blocks request from the head of our node to the block announced in the gossip sub, thus creating too many overlapping requests across multiple batches, leading to the issue reported by community #2954.

This PR changes the following:

• Moves the processing of block announcements to the end of the poll function, prioritizing finishing the ongoing missing block request results.
• Detaches pruning from the block announcements, so we prune before adding more data to our structs and so we always prune regardless of an announced block arriving.

This PR no longer restricts the request missing blocks to the current batch. The following changes were dropped in favor of tighter rate limiting on the gossipsub #3008:

• Only allow concurrent missing block requests within the current batch, thus diminishing the DoS surface significantly.
• Any block on a future batch gets buffered instead, but no request missing blocks is sent over the network.

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.

[paberr]

lgtm