net: tcp: Implement randomized retransmission timeouts

It can happen that two similar stacks enter a retransmission cycle
due to a packet collision. If the transmission timeout is the same
both stacks will retry at the same moment resulting in another
collision.

By introducing a randomized retry timeout, the chance of
a second collision is reduced and it reduces furter the more
retransmissions occur.

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