backport PR #39 to 201811 branch.
this PR to enhance xcvrd to be more robust by handling the more new system-level event:

1. wait for a certain period on system_not_ready event to overcome race condition when xcvrd started but system not ready.
2. start to routing work on system_become_ready event, this event indicates that the system is ready for SFP change event report.
3. exit on system_fail event
4. make xcvrd handle signal correctly