Introduce SLT_Timestamp mechanism for request handling timing information.
Reduce logged timer precision to microseconds
Backend fetch timestamps
Timestamp documentation
Add an XXX comment about turning off unnecessary timestamps
Fix test cases to pass with timestamp records present
Timestamp test case
Add std.timestamp() method
Update varnishncsa to read timing data from SLT_Timestamp records.
Update varnishhist to read ttfb from VSL_Timestamp record.

Fixes: #1460
Restore default_ttl to 120 seconds.

This was reduced to 20 seconds in 6d0c94c34c0e471b12982b207857934ec4e6ea57.

The git commit message does not mention default_ttl, and since this was a
fairly big commit, I don't believe the change was intentional.
Reformat to <=80 cols.