Improve TCP timestamps

[tuexen]

Summary

TCP timestamps leak the uptime in milliseconds. To avoid this, initialise the TCP timestamp with a value of a keyed hash function, which takes the source and destination IP address and the source and destination port number.
Use the same keyed hash function as used for selecting the initial TCP sequence number.

Test Plan

Ensure that the TCP timestamp is increasing over multiple TCP connections using the same 4 tuple. Check this when using the syn cache and syn cookies.