First class migration support for switching a projection from Inline to Async

[jeremydmiller]

Here's the deal:

• You have a projection that today runs as Inline, but you have ascertained that it would be better if the projection runs as Async
• Upon restarting the app -- and it's not going to be possible to do this blue/green -- Marten should start processing this projection from the current high water mark and run normally from there
• Make sure that Marten only starts the projection at the "highest" point only the first time it starts up as async