As some of the nodes can lag behind, the system can be in state, that those
nodes will send inactive (also destroyed) slave publish information. Before
publishing it to master, check if each of slaves is really present on master.

Tasks:

 - [x] prove it really works on simulated environment
 - [x] check impact on massive simulated environment
 - [x] cover with a test (optionally)
 - [ ] check test results with this change

/reviewed-on nexedi/slapos!519