Expand placement policy with subnetwork field

[alexvanin]

The best way to filter storage nodes of subnetwork is to define subnetwork ID directly in placement policy.

UPD 16.11 @cthulhu-rider

1. Define refs.SubnetID type with single numerical field and describe:

• string format;
• binary format.

Null value should refer to zero subnet.

2. Add subnet field to PlacementPolicy message. Write the docs how it affects placement building.