phase-shelve: allow for opt-out of phase-based shelves in config

This chain builds on the work in the phase-shelve topic (!194 (merged)) to add a configuration knob to disable phase-based shelve storage. As our org was considering rolling out the internal-phase, we realized that a repo may have the internal phase enabled, but it may still be desirable to switch off phase-based shelves. This chain adds a configuration shelve.store, which can be set to "internal" or "strip", to indicate which technique to use at run time.