Also, have valid_chain_p return boolean, add some debug logging and
detect invalid signature types instead of crashing.

That's for distributed nodes and we don't do that.

This filters out certificates with signing algorithms that we can't
handle.

Also, make unit tests better.

Trying to figure out why public_key:verify isn't found in docker images.

This code is untested, don't use it for now.

This might fix a weird error function_clause crash in ct/v1/add-chain.
'applications' might still not be complete.