mem-pool: Pass the remote IKE address, to re-acquire() an address during reauth

With make-before-break IKEv2 re-authentication, virtual IP addresses must be
assigned overlapping to the same peer. With the remote IKE address, the backend
can detect re-authentication attempts by comparing the remote host address and
port. This allows proper reassignment of the virtual IP if it is re-requested.

This change removes the mem-pool.reassign_online option, as it is obsolete now.
IPs get automatically reassigned if a peer re-requests the same address, and
additionally connects from the same address and port.
testing: Add a test scenario for make-before-break reauth without a virtual IP
testing: Add a test scenario for make-before-break reauth using a virtual IP