Some simple design notes to guide people

- MGCPGrammar can parse requests and replies

- MGCPCommand base class of commands
   - asDatagram to create message

- MGCPTrunk a trunk with a destination and nr. endpoints
   - MGCPVirtualTrunk for 1@mgw.. in hex
   - MGCPE1Trunk for ds-1/... handling
   - both hold MGCPEndpoints

- MGCPTransaction a command to create a transaction
   - callback for completion (wth response)
   - callback for a timeout
   - created with an endpoint and the callagent

- MGCPCallAgent serves the Call-Agent port
   - Generates the transaction id for a transaction
   - Does the retransmission
   - Handles timeout
   - Recycles transaction after three minutes
     (to block a transaction identifier)