Please merge topic/johanna/filter_subnet_table
This branch adds the filter_subnet_table bif. This bif works similar to the matching_subnet bif. The difference is that, instead of returning a vector of the subnets that match, we return a filtered view of the original set/table only containing the changed subnets.
The branch also fixes a small bug in TableVal::UpdateTimestamp (ReadOperation only has to be called when LoggingAccess() is true).