Validators at height

[iansuvak]

Why this should be merged

Closes #501 as well as adds the proposerHeightCache to get a best guess height for which to request validators.

How this works

• Accepts pchain height as an argument to createSignedMessage.
• Adds proposerHeightCache that updates itself on a set interval

How this was tested

• Adds new unit tests for proposerHeightCache
• existing tests should pass

How is this documented

Comments

[bernard-avalabs]

A few minor questions mostly to improve my own understanding.

[geoff-vball]

I didn't look over the cache implementation too hard, but looks good. The rest LGTM aside from existing comments

[iansuvak]

This doesn't need to be a hashmap. Should look into a dequeue.

[minghinmatthewlam]

no need to set a new variable here instead of usual err := assignment, will only pass line 231 if err = nil.

[iansuvak]

I'm overriding the passed in height if it's 0 so I do need the err defined prior to that so that line 232 is pure assignment to already declared vars.

I could declare a new variable to replace height instead but I think this is cleaner

Also hi 😄

[michaelkaplan13]

With ACP-151 now being implemented, do we want to close this PR in favor of adding a GetProposedValidators RPC method to the P-Chain in AvalancheGo and leveraging that instead of GetCurrentValidators or GetValidatorsAtHeight?

I think this iteration of the design was super helpful to think through and conceptualize, and that it led us to design that both simpler and more robust.

[iansuvak]

Closing in favor of #555