db/gdbm: Convert to use BuxtonHashmap

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>
db/memory: Convert to BuxtonHashmap

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>
Ensure db backends are correctly linked

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>