Skip, and log, cassette messages that are out of order
Let replay-since look for the first module with data points.

Modules with a :replay hook can return nil to mean "I don't know what
this is", or an empty sequence to mean "I know what this is, and
it's empty".
Add a :clear handle for cassette module.

Right now :clear is only used when replaying into an existing source,
so this won't be called, but I'd like to add a way to ask for some
cassette files to be removed.
Add a /clear endpoint for purging storage