Redesign resource managers implementation

[LCaparelli]

We should redesign resource managers implementations to make better use of first-class citizenship of functions in Go. This would allow us to concentrate much of the manager-specific business logic during its initialization (opposed to spreading them across multiple methods).

This could possibly also allow us to raise any necessary errors during initialization and remove errors from other methods of the Manager interface.

This would also allow us to control flow via iteration of functions instead of if and switch statements throughout a large portion of manager methods, simplifying the logic involved.

Should be worked on after #174, after validation logic settles.