Makes the docs a bit less wordy.

None of the data passed to a user read handler is allocated on the
heap (gc friendly).

The Status passed to the read callback is now passed by value, and
may have the value More when more transactions in the log are pending.

The e2e test now includes tests for reading data from a live log.

Written with raw grouping, so health checks really are included.

Minimizes issues with incompatible tags from different Varnish versions.
The log was generated using testbackend, and includes ESI requests
and backend health checks. The logs are also smaller now.

The response begins with '<', so it is not necessary to disable the
Varnish XML check.

And add tests for Reason.String() and tx.Reason.

Just pass around Status objects (== int8).