Scoru: filter internal messages in the arith pvm (with unique inbox)

The following discussion from !6549 (merged) should be addressed:

• @sribaroud started a discussion: (+5 comments)

  Because you remove this check pvms are going to process all internal messages.

  For wasm I guess it's the job of the kernel but for arith it would be nice to update it to make sure it only processes his?

  It's about to be removed, but I like the arith pvm, snif!