/reviewed-on nexedi/caucase!3

Maybe name resolution is flapping, or server is unreachable or
misbehaving... This must not cause caucase-update to exit.

So they can be customised in subclasses.

So that they do not hardcode the class to instanciate.
This prepares further class tweaking.
Also, update users.

Work around a python bug (both present in 2.7.15 and 3.6.6) which prevents
changing the ssl context on a listening socket.
Reported: https://bugs.python.org/issue34747

The code could actually not loop. Still check that deadline is not reached
when entering, as code often uses "now" as initial deadline.
Also, clarify dosctring.

Do not wait forever so test does not block.
Always clear event after it occurred.
Also, ensure itsUntilEvent behaviour is to raise so server actually stops
in _stopServer.

So the instance is readily reusable and less likely to be misused.

Spawned thread is not returned.

Except it is not present on caucase root object.
This reverts commit b26eeb38.

It is not in python3.

To ease debugging.

The intent was getting a nice error message if file was not readable, but
it causes a resource warning in python3 (file object being garbage-
collected while open - wasn't that the beauty of automatic garbage
collection to begin with ? It makes sense for writeable files as not
closing may cause race conditions, but for read-only it's just annoying).

Concatenate potential homonymous headers following specification.

Escape all quoted strings.
Add referrer.
Add user-agent.

Include both caucase name and its current version number.
Add versioneer for version number introspection, producing egg version and
caucase.__version__.

More realistic than checking server certificate.

Reference value is pre-timeshift, so it will immediately differ but for the
wrong reason.

sqlite does not allow controlling creation mode, so create the file
ourselves so it gets created when missing.

Rather than starting to listen on http before https.
This makes tests more reliable, as they will no actually wait for caucased
to be fully ready, in turn making shutdown more reliable.

It is not expiration which is disabled, but pruning from database.

For consistency with other places in caucase.

No certificate is needed to be an anonymous client, only up-to-date CA and
CRL are needed to validate service certificate.