Clustering for secp256k1 signatures verification

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Following #20 where I've been able to remove a bottle neck and increase the replay speed, I believe the next big step will be to enable multiple CPU cores usage for the heavy-weight operations that avalon does. So basically I want to do it for the signatures verification, which represent the ultra-majority of the CPU cycles used, and should improve the replay speed a lot for people with 3 cores or more. 2 cores won't change much because mongodb's process is usually already clusterized and will make use of the 2nd core already.

This would greatly improve the replay speed, especially for people using scalable cloud solutions, where it would then be possible to increase your CPU to replay faster, and turning it down once synced.

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This won't impact the state of the chain (in theory) so it won't have to go through a hardfork either :)