Introduce server id remapping.

[sergepetrenko]

Make server ids in space _cluster local to the instance and start mapping local server ids to remote. This helps to resolve conflicts if 2 instances receive their _cluster registration from 2 different masters simultaneously and is also a prerequisite for anonymous replica implementation (#3186)

[kostja]

I think _cluster, which is a registry of all existing uuids, should stay global. But we add a new space, whihc contains the actual mapping, which is local.