nfsdcld: add function to remove unreclaimed client records

This should remove any client record that has a timestamp prior to
the given time.

Eventually, this call will need to be made cluster aware when this is
run in a clustered configuration. For now, this is only suitable for
single-host configurations.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>