It is possible that the timeout value overflows, and then, the 'now'
timestamp is always larger than the packet timeout. We need a proper
timeout detection.

- add debug domains which enable multiple debugging statements
  collectively
- add a debug domain DEBUG_ALL which enables all debugging messages
- print the debug domain in the debug message
- add some debug statements
- enable some domains

To print the domain, the debug() macro converts the domain into a
string and adds the value 6 to skip the "DEBUG_" prefix.