Clean up order validation pipeline

[albrow]

Our process for validating incoming orders is complicated, and the code that does so (meshSpecificOrderValidation, onchainOrderValidation, orderwatch.Watcher.add, etc) is spread out and somewhat inconsistent. This is largely a relic of different people contributing to different parts of the pipeline and changing requirements over time.

We should take the opportunity to clean up the validation pipeline and use consistent rules for all the functions involved. For example, we should always return the same values from each function: orders that are definitely valid/accepted, those that are definitely invalid/rejected, and orders which are possibly valid and should continue through the pipeline.