Engine API: narrow down fcU skip conditions

[mkalinin]

Considering the following EL block tree:

CA <- A1 <- A2 <- HEAD
  \
   <- B1 <- B2 <- B3  

Current spec allows to skip processing in following cases:

• fcU(head: A2.blockHash), fcU(head: A1.blockHash) -- ancestors of the head of canonical chain
• fcU(head: B2.blockHash) -- ancestor of a tip of another chain (not a leaf)

After this change, the spec disallows to skip fcU(head: B2.blockHash). For instance, when BeaconBlock(B3) appeared to be invalid but the B3 itself is VALID, and then the network re-orgs to BeaconBlock(B2) the EL must handle such a re-org properly and start a build process if requested.

TODO:

• add Hive test, Import and re-org to previously validated payload on a side chain txrx-research/TestingTheMerge#6

Thanks to @rjl493456442 for helping to identify this issue.

H/t @djrtwo for the review