Low: attrd: Increase log level of first update of attribute

We are monitoring the log of change of specific attribute's value which ping RA etc. update.
Since the log of 'update' and 'delete' is LOG_INFO,
```
Feb 27 18:16:51 bl460g1n6 attrd[3885]:     info: attrd_peer_update: Setting pingd[bl460g1n6]: 100 -> 0 from bl460g1n6
Feb 27 18:16:58 bl460g1n6 attrd[3885]:     info: attrd_peer_update: Setting pingd[bl460g1n6]: 0 -> 100 from bl460g1n6
Feb 27 18:17:06 bl460g1n6 attrd[3885]:     info: attrd_peer_update: Setting pingd[bl460g1n6]: 100 -> (null) from bl460g1n6
```
We want log of 'first update' of LOG_INFO. By this patch, the log of 'first update' will be as follows.
```
Feb 27 18:16:33 bl460g1n6 attrd[3885]:     info: attrd_peer_update: Setting pingd[bl460g1n6]: (null) -> 100 from bl460g1n6
```