[plugins] director: add locking around map for roadwarrior backends.
[http] HttpWorker: wraps worker's ev loop around a mutex.
[http] HttpServer: defer ev-loop creation into HttpWorker's ctor for non-main workers
[base] DebugLogger: make it thread-safe
[base] Queue: adds compile-time option to be non-lock-free (defaults to lock-free though)
[plugins] director: make more use of std::unique_ptr<T> (DirectorPlugin)
[plugins] director: make more use of std::unique_ptr<T> (RoadWarrior)
Revert "[plugins] director: attempt to fix fcgi client abort causing backend objects to hang"

This reverts commit 4ce1ba6c75177b453e73274cb558d29db9ce618d.

It seems to make problems. So let's fix the fix right after this revert.