Closes#4328
When TrustedHeader(height) is called, if the height is less than the trusted height but the header is not in the trusted store then a function finds the previous lowest height with a trusted header and performs a forwards sequential verification to the header of the height that was given. If no error is found it updates the trusted store with the header and validator set for that height and can then return them to the user.
Commits:
* drafted trusted header
* created function to find previous trusted height
* updates missing headers less than the trusted height
* minor cosmetic tweaks
* incorporated suggestions
* lite2: implement Backwards verification
and add SignedHeaderAfter func to Store interface
Refs https://github.com/tendermint/tendermint/issues/4328#issuecomment-581878549
* remove unused method
* write tests
* start with next height in SignedHeaderAfter func
* fix linter errors
* address Callum's comments
Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>