Add support for querying network timestamps

Add creation and modified timestamps when creating a new network, and
extend the available query fields for networks with these fields,
namely 'ctime' and 'mtime'.

Signed-off-by: Christos Stavrakakis <cstavr@grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>