优化设备状态保持，自动记录心跳间隔，三次心跳失败则设备离线，不在使用设备有效期字段作为唯一判断标准，提高容错能力和稳定性。

src/main/java/com/genersoft/iot/vmp/storager/dao/DeviceMapper.java

@@ -61,6 +61,7 @@ public interface DeviceMapper {
                 "expires," +
                 "registerTime," +
                 "keepaliveTime," +
+                "keepaliveIntervalTime," +
                 "createTime," +
                 "updateTime," +
                 "charset," +
@@ -88,6 +89,7 @@ public interface DeviceMapper {
                 "#{expires}," +
                 "#{registerTime}," +
                 "#{keepaliveTime}," +
+                "#{keepaliveIntervalTime}," +
                 "#{createTime}," +
                 "#{updateTime}," +
                 "#{charset}," +
@@ -117,6 +119,7 @@ public interface DeviceMapper {
                 "<if test=\"online != null\">, online=${online}</if>" +
                 "<if test=\"registerTime != null\">, registerTime='${registerTime}'</if>" +
                 "<if test=\"keepaliveTime != null\">, keepaliveTime='${keepaliveTime}'</if>" +
+                "<if test=\"keepaliveIntervalTime != null\">, keepaliveIntervalTime='${keepaliveIntervalTime}'</if>" +
                 "<if test=\"expires != null\">, expires=${expires}</if>" +
                 "WHERE deviceId='${deviceId}'"+
             " </script>"})