rpcserver.py: perf_counter_ns is Python 3.7+
perf_counter_ns is only available in Python 3.7 and later.
Define a lambda for 3.6 and lower.
Fixes: https://pagure.io/freeipa/issue/8891
Signed-off-by: François Cami <fcami@redhat.com>
Reviewed-By: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>