Ignore expired listings

This patch adds an option to ignore expired listings.
It is required if you create a custom incremental query which will not return the entire set of available listings, such as incremental imports of only the listings updated since the last successful import.
I use a separate connection with different credentials mapped to the same Drealty property types to fetch a complete list of all available listings and set it to delete expired listings.

The secondary connection does not contain all fields, which means the hash would not be the same as in the first connection, but that is worked around using another patch which disables field mappings.