Database read throttle (#242)

Track current outstanding read requests to the database. When the configured limit is exceeded, reject new RPCs and return rpcTOO_BUSY

src/backend/BackendInterface.h

@@ -7,10 +7,12 @@
 #include <backend/Types.h>
 #include <thread>
 #include <type_traits>
+
 namespace Backend {
 
 class DatabaseTimeout : public std::exception
 {
+public:
     const char*
     what() const throw() override
     {
@@ -30,9 +32,10 @@ retryOnTimeout(F func, size_t waitMs = 500)
         }
         catch (DatabaseTimeout& t)
         {
-            std::this_thread::sleep_for(std::chrono::milliseconds(waitMs));
             BOOST_LOG_TRIVIAL(error)
-                << __func__ << " function timed out. Retrying ... ";
+                << __func__
+                << " Database request timed out. Sleeping and retrying ... ";
+            std::this_thread::sleep_for(std::chrono::milliseconds(waitMs));
         }
     }
 }
@@ -342,6 +345,9 @@ public:
     virtual void
     close(){};
 
+    virtual bool
+    isTooBusy() const = 0;
+
     // *** private helper methods
 private:
     virtual void