* wip
* sync sort of works now
* get rid of the deadlock during compaction
there is *always* a deadlock in there when we make changes like this...
* cleanup how we rebuild the sync MMR on init
* cleanup rewind logic
* roll the "fix invalid root" changes into this PR
* move rebuild_height_pos_index into txhashset
and pass in header_pmmr for header lookups
* cleanup and remember to setup sync head on init
* cleanup unnecessary ref muts
* rebuild_height_pos_index when writing txhashset
* add feature for height locked kernels
* add function to compute kernel features appropriate for lock height, and use it
* only sign kernel-features relevant fields; refactor Features
* simplify invalid kernel logic
* remove unused height arg to reward::output and run some rustfmt
* replace nested if/else by match
* introduce rewindable_kernel_view
cleanup header in extension
* cleanup and docs/comments
* txhashset does not need to be mutable here
* pull validate_kernel_history out into fn
