setdiscovery: delay sample building calls to gather them in a single place Some of the logic around sample building is duplicated in the sample builders, it would clean up thing to extract it in the top function, but this requires all codes to be in the same place. This changeset mostly exists to make the next one more clear.

setdiscovery: drop unused 'initial' argument for '_takequicksample' There is a single call site, and it is always using 'initial=True'. So we just drop the argument and the associated condition.

readmarkers: add a SHA256 fixme note

readmarkers: fast-path single successors and parents This gives about a 5% performance bump.

readmarkers: promote global constants to locals for performance

readmarkers: drop a temporary

readmarkers: read node reading into node length conditional This removes some conditional assignments