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
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