Minimal alternate metadata type support

[hannahhoward]

Goals

Avoid a future upgrade to metadata types by supporting simple handling in the client

Implementation

• Uncomment remaining types on metadata
• Add simple handling in the client for DuplicateNotSent and DuplicateDAGSkipped -- while these two things very different things than Present/Missing:
  • DuplicateDAGSkipped can essentially be treated with the new ReconciledLoader like a Missing -- just switch to a local loading only mode till you exit this branch.
  • DuplicateNotSent can be treated exactly like Present with no block contained in the message, with one caveat: it treats the block as a duplicate even if it is contained in the message. There are certain scenarios (using ExtensionDeDupByKey for example) where this may be useful information that helps us avoid holding a received block in memory longer than needed. (I am still open to the idea though that we should merge this w/o DuplicateNotSent and not add this until there's a super clear use case why we actually need this an not just imply it via the presence of a block in the message)
• By only doing this super minimal client implementation:
  • We avoid downgrading compatibility issues with the v1 protocol till its deprecated
  • We can implement a server version and possibly upgrade the client in the future (for a client that also skips duplicate DAGs), but continue to talk to old clients with no change.

[rvagg]

that extensible reconciledloader test suite is pretty neat, good that it can be so easily adapted to test these

re not including DuplicateNotSent - I think that's your call, it's not clear to me that it's going to be essential, maybe it'll be helpful, or maybe by the next time we can ship a breaking change we want to remove it.

[willscott]

@rvagg you should feel free to merge with you comment changes if you want to build off of this in the next couple weeks