Refactor Lease Expiry in Server

[timkaye11]

When a new grid server starts up, we end up waiting for the previous lease to expire. This requires something like :

select {
case <-time.After(leaseDuration):
case <-c.Done():
       return
}
err = server.Serve(lis)

otherwise, we hit get an ErrAlreadyRegistered here https://github.com/lytics/grid/blob/b80e7e722aa8365601aeca828967eab0d63129a6/registry/registry.go#L406-L414

Instead, if we had something like server.WaitforLeaseToExpire(), we could avoid the select block.