reclaim resource when failed to sending it
using switch clause to ease addition of internal message type handler
move internal used definition to process.c
using monitor_t to represent monitor reference
using `source` field in message to determine where retired message come from
rewrite inter-process monitor/demonitor using message
Merge branch 'monitor_refactor' into develop
to fix bug: monitored process hold back reference to
retired supervisor process, prevent it from deleting