MXS-1786: Fix hang on COM_STATISTICS

The commands needs to be handled separately from the rest of the result
types.

Added a test case that reproduces the problem and verifies that the change
in code fixes it.

maxscale-system-test/mxs1786_statistics.cpp

@@ -0,0 +1,24 @@
+/**
+ * MXS-1786: Hang with COM_STATISTICS
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    test.maxscales->connect();
+
+    for (int i = 0; i < 10; i++)
+    {
+        test.set_timeout(10);
+        mysql_stat(test.maxscales->conn_rwsplit[0]);
+        test.try_query(test.maxscales->conn_rwsplit[0], "SELECT 1");
+    }
+
+    test.maxscales->disconnect();
+
+
+    return test.global_result;
+}