Split Table.fold_over_recent into fold_over_recent and fold_over_deleted

This allows fold_over_recent to have the same signature as the other
functions with the same name. Tracking of deleted references (without values)
is specific to the Table.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
fold_over_recent have the same signature, so put in the common definition

Signed-off-by: David Scott <dave.scott@eu.citrix.com>