http: replace WorkQueue and threads handling for ThreadPool

Replace the HTTP server's WorkQueue implementation and single threads
handling code with ThreadPool for processing HTTP requests. The
ThreadPool class encapsulates all this functionality on a reusable
class, properly unit and fuzz tested (the previous code was not
unit nor fuzz tested at all).

This cleanly separates responsibilities:
The HTTP server now focuses solely on receiving and dispatching requests,
while ThreadPool handles concurrency, queuing, and execution.
It simplifies init, shutdown and requests tracking.

This also allows us to experiment with further performance improvements at
the task queuing and execution level, such as a lock-free structure, task
prioritization or any other performance improvement in the future, without
having to deal with HTTP code that lives on a different layer.

src/httpserver.h

@@ -159,15 +159,6 @@ public:
  */
 std::optional<std::string> GetQueryParameterFromUri(const char* uri, const std::string& key);
 
-/** Event handler closure.
- */
-class HTTPClosure
-{
-public:
-    virtual void operator()() = 0;
-    virtual ~HTTPClosure() = default;
-};
-
 /** Event class. This can be used either as a cross-thread trigger or as a timer.
  */
 class HTTPEvent