Rejig contexts to allow for processor and handle scoped objects

[calpaterson]

We want a way for people to scope certain things to

• the processor's lifecycle (example: database connection pools)
• the message's lifecycle (example: an individual database connection)

In order to do this:

• Rename the current HandlingContext to ProcessingContext to indicate that it's per-processor
• Create a new HandlingContext for each message
• Create new decorator functions for them: @proc.before_process(proc_ctx) @proc.after_process(proc_ctx) @proc.before_handle(proc_ctx, handle_ctx) and @proc.after_handle(proc_ctx, handle_ctx)