Ability to query by "updated"

[PeterParslow]

#48 addresses "harvest" in the CSW "push" sense - pushing records into another catalogue.

This discussion is for the opposite: where one catalogue polls another to "harvest" records from it. For efficiency, it would be useful to be able to query on 'metadata change timestamp' to be able to say "give my everything since last week" (when I know I last asked).

This issue asks for "updated" to be a queryable.

In DCAT, Catalogue Record -> dcterms:modified. In ISO 19115-1 terms, MD_Metadata/dateInfo . Specifically, the most recent dateInfo.CI_Date. (In ISO 19115:2003/19139:, this was gmd:Metadata/gmd:dateStamp)

q=datetime is for core schema "time", the temporal extent of the dataset that the record describes (as is DCAT Catalogued Resource 'update/modification date' dcterms:modified / ISO 19115-1 EX_TemporalExtent