HA peer should drop leases not present on the partner during sync
Let's suppose there are two HA peers A and B. The peer B dies. While the peer B is offline, the admin sends lease4-del command to the A. The peer B starts up and synchronizes its lease database with A. It correctly adds new leases and updates existing leases based on the list received from A. However, it doesn't remove the lease deleted on A while it was offline. The server admin would need to send lease4-del command to B to remove the lease.
In order to address this problem we have to fetch all leases from the B's backend and iterate over them to see if they are also present on A. In order to do so, we will have to keep the local copy of leases received from A. For Memfile, MySQL and Postgres we could do it more efficiently by comparing ranges of leases as they are ordered by IP addresses. After comparing a range of leases we could simply drop the local copy of the lease ranges. However, this won't work for Cassandra which returns leases out of order. In the Cassandra case we will have to collect all leases returned by the peer.