nlbwmon: fix db cleanup

[feckert]

@jow- The database will only be cleaned up when the daemon is restarted. With this change, each refresh of the database checks whether the artifacts should be cleaned up from the past database. Thus, no more than configured are kept.

This fixes https://github.com/openwrt/packages/issues/14297

[jow-]

You should only call database_cleanup() if the earlier database_archive() returned -ESTALE, only then there's anything to potentially clean up.

[feckert]

@jow- ping

[schuettecarsten]

@jow- ping... :-)