Bug: Prism plugin blocks during long-running process

[farscapian]

Let's say you have a 7-member prism that has a bunch of tor-based destinations. Executing this prism through invoice_payment can take a while, minutes or more. Well let's say during that execution, you want to run another RPC method that is not long running (e.g., binding-list). Binding-list blocks. Bummer.

I think this means that we'll need to separate invoice_payment event handling logic into a distinct plugin.