Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

That way, each plugin can decide to implement it better.

Prepare to allow more complex expressions.

This enforces the "not nullptr" rule.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

Implementations shall use exceptions instead.