The native invocation in Read is locked per Stats object, since
VSC_Iter is not safe for concurrency with the same vsc and vsm
handles.

This is not necessary for the descriptions call, since it is only
invoked once per object at Attach time.

Use sync.Map for the global callback maps, and use 64 bits of
secure randomness for their keys (using the map length to generate
new keys was not safe for concurrency).

In rare cases, VSC_Iter may pass NULL as the pointer to a point,
so just skip it if that happens.

Also rename some of them with camel case.

And refactor the testing code to re-use results of a scraped log.

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.