[replication] Storing of modified entries can be skipped in full syncrepl mode

When we have replication setup, we do copy all the modified entries into a btree for each replica we are dealing with.

We can avoid doing this as when a replica will reconnect, it will send the latest EntryCSN it has received, and we can now locally find all the modified entries by comparing the received entryCSN with the local entryCSNs using the index, and send all the updated entries to the replica.

Although we can still keep a track of all the deleted entries (storing their DN and entryCSN) so that w can send the deleted entries to the replica, instead of asking the replica for all its DN and compute the deleted entries with that set. We have to be careful though : the deleted entry should be sent in the right order, within the modified entries.