add(): Stop deleting an existing cookie when storing an expired cookie.

This is a backout of what I've done previously.  Expiration check is
better done in extraction where a user can optionally specify a base
date.

Add an internal method delete() to delete an existing cookie.
each, cleanup: Fix the base date for expiration when the method starts.

This change is to remove an assumption that the clock is monotonic.
Add MozillaStore.
Add sqlite3 as runtime dependency, or MozillaStore.
Make GC threshold user specifiable.
Bump VERSION to 1.0.0.pre3.