This greatly helps with memory management on applications that
may want to daemonize and open/close database several times.
Also the lifetime and "owner" of memory for all data is now
explicitly bound to owning struct apk_database, which might
be helpful when writing language bindings. As side effect, the
interned "atoms" are unique only within what apk_database, so
comparing packages from different apk_database may not work
as expected.
Fixes#10697
Instead of having a null pointer, use a dummy array which just
says the array is empty. This helps in multiple places of the code
which would otherwise need explicitly need to check first if the
array exists. This has been cause of multiple seg.faults in the
past as the array check is easily omitted.
This also removes (or fixes) all existing checks accordingly.
Timo Teräs