initial split of database to a separate crate

It should reduce compile time / memory usage to put quite a bit of the code
into a separate crate. I also intend to limit visibility of some things to
only within the db crate, but that's for a future change. This is the smallest
move that will compile.

src/cmds/ts.rs

@@ -28,8 +28,8 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+use db::recording;
 use failure::Error;
-use recording;
 
 const USAGE: &'static str = r#"
 Usage: moonfire-nvr ts <ts>...