Inbox: Make `max_inbox_size` a protocol constant

The following discussion from !4020 (merged) should be addressed:

• @hans.hoglund started a discussion: (+1 comment)

  We also need to check that max_inbox_size is not exceeded. Ofc max_inbox_size does not exist yet: it should probably be a protocol parameter.

  We could also make a ticket for this work and merge as-is.