Create clean exit path for traffic_manager on SIGTERM

[jeredfloyd]

Since the removal of traffic_cop, it appears that there is no way to cause traffic_manager to exit cleanly. SIGTERM causes the cleanup of child processes and a graceful shutdown, however still exits with code 15. We can ignore this in the systemd unit file, but it's cleaner to have some path to a clean exit of traffic_manager.

[jeredfloyd]

It might also be desirable to add a "--manager" option to traffic_ctl server stop, to be parallel with restart? I'm not entirely clear on the intended operation, however both the init.d file and the systemd unit use signals today, so a clean way to exit traffic_manager via signal is always valuable.

ETA: Looking through the code further, the command traffic_ctl server restart --manager actually leads (confusingly) to a clean shutdown of traffic_manager (and not a restart, as suggested). There's no way that a TS API command could restart the traffic_manager process without the cooperation of the outside starting mechanism (init or systemd), so this traffic_ctl command should be removed.

The underlying state enum "MGMT_PENDING_RESTART" could be renamed to "MGMT_PENDING_EXIT" (STOP is used for stopping the traffic_server process only) and the plumbing then corrected on the traffic_ctl side to move the "--manager" option from the restart command to the stop command. This would fix the confusing behavior for that command today -- I can do that in another PR.

The above commentary is independent of this PR -- a signal is still desired!

[bryancall]

@jeredfloyd We do plan to get rid of traffic_manager in the next major release (10.0.0).

[jeredfloyd]

@bryancall Thanks! Once I have 9.x in Fedora I will try to follow future plans a little more closely.

[zwoop]

Cherry-picked to v9.2.x