Convert tcp_now() time counter to 64 bit.

Convert tcp_now() time counter to 64 bit.

After changing tcp now tick to milliseconds, 32 bits will wrap
around after 49 days of uptime.  That may be a problem in some
places of our stack.  Better use a 64 bit counter.

As timestamp option is 32 bit in TCP protocol, use the lower 32 bit
there.  There are casts to 32 bits that should behave correctly.

Start with random 63 bit offset to avoid uptime leakage.  2^63
milliseconds result in 2.9*10^8 years of possible uptime.

OK yasuoka@