Service Installer Cooperative Drain Upgrade w/o Reboot

[planetchili]

System for enabling upgrades of shared service without rebooting.

• Service publishes client activity state via service state system
• Service exposes drain command via custom control code
• Installer issues drain command
• Service requests connected clients to drain via subscribed event channel
• Service creates and gives clients a handle to event notifying when to reconnect
• After all clients disconnect or timeout, service forcefully close remaining sessions and set drained status
• Installer polls and detects drained status, stops service replaces and starts it again
• Service, after initializing, opens existing event object and signals reconnect availability
• Clients reconnect