Ref: #8983

The previous value was quite bogus.

This new implementation opportunistically tries to read data before
invoking poll(). This reduces latency when data is already waiting in
buffers, allows receiving pending data in non-blocking fashion even if
the object has been killed. This also gives GnuTLS a chance to dequeue
data from internal buffers.

Also the corner case of 0 read should now be handled correctly.

The corner case of an error after success read is no longer handled.
This was race-prone and useless.