Implement SCIM change log and expose API to get changes from certain date

[nynymike]

If you want to connect Gluu to an IDM system with bidirectional synchronization,
there is no problem writing changes into Gluu but it is hard to read
changes from Gluu into the IDM. The reason is that you simplly do not know what
has changed since the last time you searched Gluu.

So the API would provide a search "return all
entries where modifyTimestamp is newer then X" the IDM could
remember the highest timestamp of all entries returned by the last
search and start the next search at this point.