mem-pool: add option for reusing online leases, and disable it by default

mem-pool: add option for reusing online leases, and disable it by default

Mainly for reauthentication with third party implementations, we allowed to
reuse an online lease, but only for the same peer identity and when it
explicitly requested the same address.

This has always been problematic, because it changes the reqid of the CHILD_SA
with the same traffic selectors, breaking the old tunnel. As we now reject
such policy overwrites, this usually lets the installation of the new policies
fail. We therefore disable reassignment of online leases by default.