overhaul "TT" (TransportToken) naming convention

TTID: Transport Token Identifier, allocated by mailbox, shared with
      recipient for retrieval purposes
TT: Transport Token, created by mailbox, given to recipient, used to mint
    a new STT for each sender
STT: Sender Transport Token, created by recipient, given to sender as
     part of the Sender Transport Record, used to mint a new MSTT for
     each message
MSTT: per-Message token, created by sender, put in each message to inform
      the mailbox which transport the message is arriving on

This leaves lowercase "tid" to refer only to the integer AUTOINCREMENT
database index of the "mailbox_server_transports" table, used by the
server to track which transport (customer) the message is being
delivered to.