net: tcp: Implement a fast retransmit algorithm

Instead of waiting for the retransmit timeout, retransmit as soon as
missing data is deduced based on a triple-duplicate ACK.

Increase the number of buffers in the testcase, to allow for at least 4
packets in flight to trigger the triple-duplicate ACK.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>