Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/httpserver.cpp

@@ -79,7 +79,7 @@ private:
     {
     public:
         WorkQueue &wq;
-        ThreadCounter(WorkQueue &w): wq(w)
+        explicit ThreadCounter(WorkQueue &w): wq(w)
         {
             std::lock_guard<std::mutex> lock(wq.cs);
             wq.numThreads += 1;
@@ -93,7 +93,7 @@ private:
     };
 
 public:
-    WorkQueue(size_t _maxDepth) : running(true),
+    explicit WorkQueue(size_t _maxDepth) : running(true),
                                  maxDepth(_maxDepth),
                                  numThreads(0)
     {