* grandpa: use AuthoritySetChanges to generate warp sync proof

* node: init grandpa warp sync protocol

* grandpa: iterator for AuthoritySetChanges

* grandpa: rewrite warp sync proof generation

* grandpa: remove old code for warp sync generation

* grandpa: fix indentation

* grandpa: fix off by one

* grandpa: use binary search to find start idx when generating warp sync proof

* grandpa: add method to verify warp sync proofs

* grandpa: remove unnecessary code to skip authority set changes

* grandpa: add test for warp sync proof generation and verification

* grandpa: add missing docs

* grandpa: remove trailing comma