Implement first version of HintRouter

The hintrouter is now in principle capable of routing requests
to the master or to some slave (in a round robin fashion) based
upon hints set by some earlier filter.

Note that as the router is completely oblivious of transaction
boundaries, using it with transactions and autocommit being off
will not make anyone happy.

Recognizing transaction boundaries using regexes and then pinning
the server until transaction commit would be needed.

server/modules/routing/hintrouter/dcb.cc

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "dcb.hh"
+#include <maxscale/atomic.h>
+#include <maxscale/service.h>
+
+
+Dcb::Dcb(DCB* pDcb)
+    : m_pDcb(pDcb)
+    , m_pRefs(NULL)
+{
+    try
+    {
+        m_pRefs = new int (1);
+    }
+    catch (const std::exception&)
+    {
+        dcb_close(pDcb);
+        throw;
+    }
+}
+
+Dcb::Dcb(const Dcb& rhs)
+    : m_pDcb(rhs.m_pDcb)
+    , m_pRefs(rhs.m_pRefs)
+{
+    ++(*m_pRefs);
+}
+
+Dcb& Dcb::operator = (Dcb rhs)
+{
+    swap(rhs);
+    return *this;
+}
+
+void Dcb::dec()
+{
+    ss_dassert(*m_pRefs > 0);
+
+    if (--(*m_pRefs) == 0)
+    {
+        HR_DEBUG("CLOSING dcb");
+        // TODO: You should not need to manually adjust any
+        // TODO: connections number, dcb_close should handle that.
+        SERVER_REF* pSref = m_pDcb->service->dbref;
+
+        while (pSref && (pSref->server != m_pDcb->server))
+        {
+            pSref = pSref->next;
+        }
+
+        if (pSref)
+        {
+            atomic_add(&pSref->connections, -1);
+        }
+
+        dcb_close(m_pDcb);
+
+        delete m_pRefs;
+    }
+}