Zero Downtime (ZDT) support for backend instances

This is the first of several iterations toward ZDT. The scope in this release covers backend instances without DB upgrade, wherein an active backend processes can continue to run during the upgrade process. Frontend instances and upgrades to the database are made within the standard maintenance window during which the system access is restricted. Supplementary updates containing changes only to the backend code can be applied without downtime.

This version replaces backend code using Blue or Green deployment. The process starts with creation of new group in Green (considering current running/active group is Blue). This new group can have instances with updated code. Once the new instances are ready, all new processing requests move only to instances in Green. The remaining instances from Blue are terminated once all processing on those instances is completed.