Refactor and fix process restart handling
Add EXCEPT_SYSTEM_RESTART process restart mode
Add PD Doctor role

Elected leader that monitors and manages PD and resource
health. So far it only coordinates PD initialization and
system restart.
Expose a dashi operation for setting PD boot flag
Tests and fixes for doctor and system boot
Add epu-zktool script for ZK management in boot
Refine process restart modes
Minor PD cleanups
Add cleanup option to zkcli
Move non-restartable processes to TERMINATED

under system restart
Move system restart process control to config dict

Instead of using new RestartMode values.
Fix process state docs
Clear out EPU-layer data in ZK setup step

And add tests
Rename nosystemrestart -> omit_from_system_restart
Merge pull request #4 from ooici/restart

Process dispatcher system restart support