recvbuff cleanup
In https://lists.ntpsec.org/pipermail/devel/2018-October/006721.html, @hal.murray suggested an area for future work:
The old buffer allocation pool is still there. That could be cleaned up.
refclock_generic still calls get_free_recv_buffer
There are 2 calls in ntp_io
I think they could be replaced by chunks on the stack or globals.
Checking/understanding the data input path for refclocks might be interesting.
There are 2 cases. The ACSII text mode uses readline so the driver gets a
complete message on each call. The binary mode has to take whatever data is
available and the driver has to keep collecting until it gets a whole message.