* header MMR in use within txhashset itself
works with fast sync
not yet in place for initial header sync
* add the (currently unused) sync_head mmr
* use sync MMR during fast sync
rebuild header MMR after we validate full txhashset after download
* support missing header MMR (rebuild as necessary) for legacy nodes
* rustfmt
* comments/docs
* rustfmt
* cleanup DBBackend
* cleanup DBBackend
* cleanup
* rename to HashOnly
* rustfmt
* cleanup backend.append()
* simply pmmr append api
no need to pass position when appending
* cleanup
* simplify vec_backend to match simpler append api
* rustfmt
* docs/comments
* rustfmt
* cleanup
* 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
