pingpong: stop using the download buffer

pingpong: stop using the download buffer

The pingpong logic now uses its own dynbuf for receiving command
response data.

When the "final" response header for a commanad has been received, that
final line is left first in the recvbuf for the protocols to parse at
will. If there is additional data behind the final response line, the
'overflow' counter is indicate how many bytes.

Closes #12757