joinall the greenlets 30 secs before we issue the killall cmd.

Summary:
let the greenlets finish themselves.

Test Plan: manually test it

Reviewers: lguo

CC:

Task ID: #

Blame Rev:
Merge pull request #4 from DikangGu/v0.7.0insta2

joinall the greenlets 30 secs before we issue the killall cmd.