Check file handle limit on startup (RIPD-442, RIPD-1024):

Calculate the number of file descriptors that are needed during
execution based on the configuration file, with a hard floor
of 1024, adjusting the limit if possible. Refuse to run if enough
fds are not available.

Additionally, allow administrators to limit the number of incoming
connections a configured port will accept. By default no limit is
imposed.

src/ripple/nodestore/backend/NullFactory.cpp

@@ -98,6 +98,13 @@ public:
     {
     }
 
+    /** Returns the number of file handles the backend expects to need */
+    int
+    fdlimit() const override
+    {
+        return 0;
+    }
+
 private:
 };