performance: add a verification file to the database (#14693)

Reading the database repeatedly can be quite slow.  We need a way to speed
up Spack when it reads the DB multiple times, but the DB has not been
modified between reads (which is nearly all the time).

- [x] Add a file containing a unique uuid that is regenerated at database
    write time. Use this uuid to suppress re-parsing the database
    contents if we know a previous uuid and the uuid has not changed.

- [x] Fix mutable_database fixture so that it resets the last seen
    verifier when it resets.

- [x] Enable not rereading the database immediately after a write. Make
    the tests reset the last seen verifier in between tests that use the
    database fixture.

- [x] make presence of uuid module optional