spec/ace: should clarify isolators stacking behavior

[lucab]

ACE spec defines multiple isolators without clarifying their stacking behavior. In particular, it seems that the following details are currently under-specified:

• Precedence: some isolators can be defined in both pod and app scope. Overriding behavior and precedence should be specified.
• Stacking: a single isolator type can be specified multiple times. Stacking behavior should be explicitly allowed or forbidden.
• Conflicts: some isolators combinations may make no sense (eg. retaining and revoking capabilities at the same time). Conflicts between isolators should be clarified.