This commit doesn't change any behavior, just organize the code in
different way to make it easier to implement different Criterias
to include nodes. Today we use NodeID and Subnet based selection
but later Criteria can be extended with different kind of
placement rules (like geofencing).
The change nodeselection is used by segment allocaton (upload) and repair
and excludes nodes from an in-memory selection.
Resolves https://github.com/storj/storj/issues/4240
Change-Id: I0c1955fe16a045e3b76d7e50b2e1f4575a7ff095
