Replace static monitor handler tables with dynamic handler registry

  * Keep a list of handlers as part of MONS_INSTANCE
    - each handles a command/opt_type pair
    - registered via mons_register_handler()
  * Scan the list of handlers when servicing a monitoring request
  * Add handlers for version and uptime, registered through tr_main.c
    (probably need to move these, but this works as a demo)