* 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
