Inbox terminology

[afck]

If a confirmed block sending a message has been seen in a node (client or validator), and the message put into the recipient chain's inbox, we currently call it "added" or "received". We don't have a good term for when it has been included in a confirmed block as an incoming message on the receiver chain, but in the inbox implementation itself we call it "removed".

After discussing with @deuszx, we'd suggest changing the terminology:

• If the confirmed sender block has been seen and the message added to the inbox, the message is "confirmed".
• If a confirmed block has included it as an incoming message, the message is "received".

We would change the comments, and also e.g.:

• find_received_certificates → find_confirmed_messages
• added and removed in the inbox → confirmed and received