save partition placement constraints.

[emaxerrno]

Today, the admin api allows users to specify partition placement constraints for data residency. However, in the case of total cluster failure (all nodes reboot), we will loose the constraint lists because we do not write them to ring0/controller-log.

1. we must write them
2. we must recover them in case of total collapse

[mattschumpert]

@mmaslankaprv I heard this is a misunderstanding perhaps. Should this be closed?

[mmaslankaprv]

yes, closing