Prune 'stale' storage obligations to correct host financial metrics.

This PR introduces the PruneStaleStorageObligations method. This method will delete all 'stale' storage obligations from the host database. This feature might become obsolete when the host gets an update but until that date, there are no good (reliable) financial metrics for the host.

This feature is an addition to the host module.

A test is added to show why this method is needed and to verify the working of the method.