Add cli support to clean up useless trigger events

[amewayne]

FEATURE

Summary

I deployed moira on my production environment but as time grows, trigger events is gonna run out of Redis memory. So I suggest add a command tool to clean up outdated trigger events and save memory.

Details

1. add feature in cli module, use cli --cleanupevents 2d to clean up events triggered two days before.
2. add database interface to remove those events

[amewayne]

if possible, I will work on this issue. Thanks.