op validation. Tries to validate ops by asking database_api to give the...

op validation.  Tries to validate ops by asking database_api to give the number of required keys.  In theory, a non-zero response means it's ready to sign.  If there's an exception, the op refers to consensus data that is invalid (e.g. a vote that tries to apply to an unknown permlink as "unknown key").  Problem is, it doesn't deal with public key prefixes very well.  We have a strategy for handling this before submit, so maybe that's a better place to put this kind of logic, especially since gatling should normally never worry about the target endpoint.