Consider processing a block before it has been written to the VolDB disk

@njd42 and @karknu suggested we could skip over IO latency if we processed a block before waiting for it to hit the VolDB disk.

This is the relevant code; the initial suggestion is simply to do these two steps in the opposite order.

https://github.com/input-output-hk/ouroboros-network/blob/c82309f403e99d916a76bb4d96d6812fb0a9db81/ouroboros-consensus/src/Ouroboros/Consensus/Storage/ChainDB/Impl/ChainSel.hs#L285-L295

This Issue is to consider the VolDB design and determine if that swap would be safe and whether it would indeed decrease block propagation delay.