Separate syncd service from swss service

[yxieca]

- What I did

1. Separate syncd service from swss service to become an independent service.
2. Add code to make sure syncd and swss starts/stops/restarts synchronously.
3. syncd and swss starts/stops/restarts independently if warm boot is requested.

- How to verify it

• There is not enough warm boot support to validate warm boot behavior yet.
• DUT boots up and syncd/swss both initialized properly.
• Stop/start/restart syncd, validated swss follows the state changes.
• Stop/start/restart swss, validated syncd follows the state changes.
• config load_minigraph works as expected.

[jipanyang]

Some questions:

<1> So warm restart is disabled on both syncd and swss, then "systemctl restart" either one will cause the other service to be restarted too?

<2> What if warm restart is enabled on swss but disabled on syncd, or the vice versa. Will the change handle restart of either service?

<3> If syncd is performing warm restart itself, the get operation from orchagent is synchronous, will some get request be dropped by syncd or times out in orchagent?

[yxieca]

Thanks Jipan for great questions:

<1>. Yes, with these change, syncd and swss will change state synchronously, one changes, the other will follow.
<2>. As we discussed, if swss is warm restarting, during which syncd triggers cold restart, both service should end up doing code restart. However, as we discussed, there might be specific timing scenarios that we need to consider and test.
<3>. I removed code to support syncd service warm reboot by itself.

[lguohan]

for <1>, restart syncd should not restart swss in non-warm boot scenario.
for <2>, if warm restart is enabled in swss, restart swss will not cause syncd to restart. if does not matter whether warm restart is enabled for syncd or not. if warm restart is enabled in syncd, then restart syncd will not restart swss.
for <3>, swss has get operation timeout, currently is 30 seconds. when syncd is in warm restart, we need to have orchagent wait till syncd is restarted.. currently this is not supported.

we probably should have a matrix, warm restart enabled for swss/syncd, and which service are we restarting. But, I think we should focus on making swss/syncd both warm or both cold, and test swss and syncd restart first.