Prorated validation rewards

[cam-schultz]

Why this should be merged

Fixes #526

How this works

Adds the function claimValidationRewards to PoSValidatorManager that allows rewards to be claimed from an active validation without ending the validation. There's no interaction with the P-Chain, so the core of this feature is making sure that uptimes are properly accounted for.

But how do we calculate the uptime for a floating claim interval when uptimes are provided relative to the start of the overall validation period? When processing a claim, we need to somehow offset the supplied uptime to match the period of time since the last claim. A key point is that claimValidationRewards reverts if the supplied uptime is not sufficient for rewards (i.e. is < 80%). After processing a claim, we store the claim time and the smallest sufficient uptime needed for that claim (not the supplied uptime). On the next claim, we offset the supplied uptime by this amount to normalize the uptime to the new interval since the last claim. This normalization to windows within the overall validation period yields the same uptime-based descisions since that calculation is linear. Note that If the rewards curve is also linear, then the rewards from multiple claims will match the rewards from a single claim over the same time period as well.

The uptime percentage for a given claim interval $i &gt; 0$ is

$(U_i-U_{min_i})/(t_i - t_{i-1})$

where

$U_{min_i} = α(t_{i-1} - t_{start})$ if $i&gt;1$, and

$U_{min_i} = 0$ if $i &lt;= 1$

• $U_i$ is the uptime proof supplied for claim interval $i$. This is given in seconds since the start of the validation period, not the claim interval $i$
  • $U_i-U_{min_i}$ normalizes this to claim interval $i$
• Claim interval $i$ is defined as the range $t_i - t_{i-1}$
• $α$ is the uptime threshold percentage, which is $0.8$

As an example, for $i=1$ this reduces to $U_1/(t_1 - t_0)$, which is the uptime percentage for an interval starting from the validation start time.

Finally, this change moves the uptime threshold calculation to PoSValidatorManager, since it it used to calculate $U_{min_i}$. uptimeSeconds is left in the IRewardsCalculator interface to allow for rewards curves that wish to scale rewards based on uptime.

How this was tested

CI, new unit tests

How is this documented

TODO

[cam-schultz]

Unfortunately we're bumping against the contract size limit again. A quick experiment consolidating custom errors yielded marginal gains, but changing the ValidatorMessages packing methods to external brought us back under the limit by a fair margin. Separating the large contract into multiple smaller contracts is the most scalable way to workaround the contract size limit, so this is a strategy we will likely need to employ anyway, despite the increased gas needed for delegatecall.

Draft PR here: #604