Asyncify some verifications of Blocks in the ChainSyncer

[ec2]

Issue summary

There are some checks in the ChainSyncer's validate(&self, block: &Block) -> Result<(), Error> method that could use some async.
This is a nice optimization to have, since checks don't have to happen sequentially. Furthermore, currently when one check fails, the whole method fails. This implies that we can only recover one error per block. When done asynchronously, we can pass back multiple errors, giving us a more rich error.

Some checks that can be async are:
Miner Check, Signature Check, Message Check, Winner Check

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