Sync to highest possible head given the peers available

[mcdee]

Step 2 of the initial sync process looks for a peer with a finalized epoch of the local node's latest head slot. This is okay if at least one peer is fully up-to-date, but if not it causes a problem because no peer is able to serve the node any blocks and the code stays in the loop.

This PR changes step 2 to fetch blocks from the peer with the latest finalized epoch regardless of where it is in absolute terms. This ensures that step 2 of the initial sync process will definitely be able to proceed.

(Note that there is a general issue with both old and new code that if none of the peers to which this node is connected is up-to-date the node will, unsurprisingly, fail to sync to head. This will resolve itself eventually if the peers are themselves connected to up-to-date nodes, but the node could take more active measures to find up-to-date peers. This is something to explore later.)

[codecov]

Codecov Report

Merging #4570 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4570   +/-   ##
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  Coverage   43.47%   43.47%           
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  Files         203      203           
  Lines       15376    15376           
=======================================
  Hits         6685     6685           
  Misses       7563     7563           
  Partials     1128     1128           

[stale]

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