Cleaned up use of MailAddress

Uses the slightly better MailAddress API. 
Trader's moniker and the POBox's owner are now cleanly replaced with MailAddresses.

Also fixes a couple bugs I ran into:
The Trader was not getting its moniker set on the client, only the server.
Once that was fixed, active Traders were not listed in the Catalog. Traders were registering with the PostOffice before it was ready. This fixes the issue by making it so PostOffice always has a list of active trade stations, instead of creating the list after the first request.