Drain stop and drain improve

[zizhong]

This PR includes

1. Add maximum time limit for draining from @maskit,
2. Add traffic_ctl server stop --drain from @maskit and
3. Fixing #2906: improve draining from me.

For the Fixing #2906: improve draining,

1. Adding traffic server drain subcommand
2. Adding default option, --no-new-connection option and --undo option

TODO:

1. implement is_server_idle_from_new_connection()
2. update docs
3. adding tests

@maskit @bryancall @zwoop @oknet @scw00 @masaori335 can you guys review it when you got time? Any ideas would be much appreciated!

[maskit]

Thanks for making progress on drain feature.

Some parts of the changes are written by me, but to be honest, I'm not familiar with this area. I just imitated code that is already there. I'd like traffic_manager / traffic_ctl experts to review this.

@jpeach This includes what we discussed on the list long time ago. Please review it if you have time.

[maskit]

[approve ci]

[zizhong]

The autest failure is not reachable now. Can you help to trigger another one? @maskit @bryancall @zwoop @scw00 ? Thanks!

[maskit]

[approve ci]

[bryancall]

[approve ci debian]

[zwoop]

@zizhong This is getting close, can you squash the commits into one?

[zizhong]

@zwoop Thanks. Did the squash.

[maskit]

LGTM.

As I commented on the previous PR, "--undo" looks a bit strange because of its lack of asymmetry. "start and stop" vs "drain and drain --undo". But I don't have better ideas.

Since I wrote some of the code, I can't approve this because it would be a kind of self approval.