Allow reduction in deleted item retention for high volume scenarios

[nickevansuk]

Proposal

In high volume scenarios the retention of deleted items in an RPDE feed may be reduced to reduce the number of deleted items that build up in the feed.

In the next version of this specification, we should allow this to be communicated as part of the feed meta data.

Beta adoption

For beta adoption of this reduction, suggest 2 days as a minimum for each feed (to ensure that daily harvesting still works as expected).