Remove block types with both execute and availability properties

[dapplion]

Motivation

Those where the good old days..

SignedBeaconBlock --------
         |               |
         |      GossipVerifiedBlock
         |               |
SignatureVerifiedBlock  <-
         |   
ExecutionPendingBlock
         |
        END

Where block types had a linear progression. Now there are two states progressing in parallel: execution + availability. Now block types are nested, featuring a combination of enums and names with double status

• ExecutionPendingBlock: MaybeAvailableBlock + import data
• MaybeAvailableBlock:
  • Available: AvailableBlock
  • Not available: SignedBeaconBlock
• Availability
  • Available: AvailableExecutedBlock
  • Not available: ()
• ExecutedBlock:
  • Available: AvailableExecutedBlock
  • AvailabilityPending: AvailabilityPendingExecutedBlock
• AvailabilityPendingExecutedBlock: SignedBeaconBlock + import data
• AvailableExecutedBlock: AvailableBlock + import data

Changes

This PR explores returning linearity by making availability a property of the block types, since it happens in the background.

SignedBeaconBlock --------
         |               |
         |      GossipVerifiedBlock
         |               |
SignatureVerifiedBlock  <-
         |   
ExecutionPendingBlock
         |
   ExecutedBlock
         |
ExecutedAvailableBlock

TODO

• Remove AvailabilityPendingExecutedBlock. It's existence does not affect things too much now as its main consumer (the overflow cache) is isolated.