Begin client-side implementation of buxton_*_list_keys

All this does so far is enable the client to use the function whilst
in direct mode. The client will be able to obtain a list of all keys
within the given layer.

The "glue" to make this work needs implementing within the backend
modules and then the protocol. These will follow in separate commits.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
db/gdbm: Implement list_keys

This enables clients to list all keys within a given layer, returning
a new BuxtonArray of all keys. This may return an empty BuxtonArray
if no keys are found within the layer.

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