Flash map / flash cookie support

[greenrd]

The idea is that you have ephemeral state which only exists for the next request and is then deleted. This can be used for e.g. redirecting to another page and displaying a "success" widget - the success message would be stored in the flash map / flash cookie.

A problem with this (at least, as they are implemented in Play 2.0.1) is it doesn't play well with multiple tabs being used by the same user at the same time.

Scalatra does already have a flash map implementation but I'm unclear on how this could be used from Bowler.

[greenrd]

Note: The reason why you don't just use ordinary HTTP parameters for such use cases is they're not ephemeral enough - you don't want the same message to be displayed twice.