Use automation state for devices used by api

lib/db/api.js

@@ -153,6 +153,7 @@ dbapi.saveDeviceInitialState = function(serial, device) {
   , reverseForwards: []
   , remoteConnect: false
   , remoteConnectUrl: null
+  , usage: null
   }
   return db.run(r.table('devices').get(serial).update(data))
     .then(function(stats) {
@@ -212,6 +213,20 @@ dbapi.setDeviceAbsent = function(serial) {
   }))
 }
 
+dbapi.setDeviceUsage = function(serial, usage) {
+  return db.run(r.table('devices').get(serial).update({
+    usage: usage
+  , usageChangedAt: r.now()
+  }))
+}
+
+dbapi.unsetDeviceUsage = function(serial) {
+  return db.run(r.table('devices').get(serial).update({
+    usage: null
+  , usageChangedAt: r.now()
+  }))
+}
+
 dbapi.setDeviceAirplaneMode = function(serial, enabled) {
   return db.run(r.table('devices').get(serial).update({
     airplaneMode: enabled