[core] Limit delivery box to 128 items in flight

[sruon]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Limits dbox to 128+8 slots mirroring retail.
Adds appropriate error packet when recipient dbox is full when attempting to send OR return
Adds an extra packet to dbox flows as I noticed retail did. Doesn't appear to change much though.

Notes:

• Return error packet WILL NOT PRINT AN ERROR MESSAGE - this is the client behaving as coded and is not a bug
• Unlike retail, this does not destroy AH gil and returned items because that change is a little more involved and I don't feel there's any good reason for it.

Steps to test these changes

Reduce the limit to 3 and send yourself 4 items...

[sruon]

Forgot to mention but Martel did the original testing. I don't know if they have a GitHub handle.