servers have arbitrary limits that we should stay below. to ensure this
happens use MAX_XFER_BUF_SIZE as maximum size per request. if we read
data large than that, break it apart into multiple requests
(I am mostly guessing here, the rfc doesn't seem to impose any size
constraint on the write requests themselves, so probably packet
constraints apply by default)
BUG: 404890
FIXED-IN: 20.04.2