* add segmenter for generating segments from txhashset with consistent rewind
* rework segmenter to take a txhashset wrapped in rwlock
rework our rewindable pmmr so we can convert to readonly easily
* placeholder code for rewinding readonly txhashset extension to build a rangeproof segment
* segment creation for outputs/rangeproofs/kernels/bitmaps
* placeholder segment impl
* commit
* rework segmenter to use a cached bitmap (rewind is expensive)
* cache segmenter instance based on current archive header
* integrate the real segment and segment identifier with our segmenter
* exercise the segmenter code on chain init
* wrap accumulator in an arc, no need to clone each time
Antioch Peverell
cba3137338