Expose the transaction reason through the API

This allows the parsed transaction reason to be visible in the
transactions returned from the API.

This is to pave the way for varnishncsa to see which req transactions
are ESI's and skip these.
varnishncsa: Don't output anything for a transaction that is restarted
varnishncsa: Don't output ESI subrequests
Update documentation of Begin and Link log records