implement Mem_store.get_location

There is no reason whatsoever that many of the base tests should end like this:

        Nov  8 15:31:11: info: messaging thread finished
        Nov  8 15:31:11: info: Closing store (0s)
        Nov  8 15:31:11: debug: closing store...
        Nov  8 15:31:11: info: Closing store (0s)
        Nov  8 15:31:11: debug: closing store...
        Nov  8 15:31:11: info: Closing store (0s)
        Nov  8 15:31:11: debug: closing store...
        Nov  8 15:31:11: debug: closed store
        Nov  8 15:31:11: debug: closed store
        Nov  8 15:31:11: debug: closed store
        Nov  8 15:31:11: fatal: going down: Failure("not supported")
        Nov  8 15:31:11: fatal: going down: Failure("not supported")
        Nov  8 15:31:11: fatal: going down: Failure("not supported")
        Nov  8 15:31:11: fatal: after pick
        Nov  8 15:31:11: fatal: after pick
        Nov  8 15:31:11: fatal: after pick
        Nov  8 15:31:11: info: Not dumping state
        Nov  8 15:31:11: info: Not dumping state
        Nov  8 15:31:11: info: Not dumping state

Why that fatal error doesn't result in the test failing is another interesting question, which will not be answered with this branch/PR.
I do want to find out and have put it on the list of TODOs I'm maintaining.