Add support for filters implemented in C++

C++ header files have the .hh-suffix to distinguish them from C
header files, but also to allow a C++ header file (e.g. xyz.hh) for
an existing C header file (e.g. xyz.h).

- cpp.hh        : Basic C++ stuff. Introduces the namespace "maxscale"
                  and its synonym "mxs".
- filter.[hh|cc]: Template class Filter and companion class FilterSession
                  to be used when implementing filters in C++.
- spinlock.h    : Wrapper and lock guard class for SPINLOCK.

server/core/CMakeLists.txt

@@ -1,4 +1,4 @@
-add_library(maxscale-common SHARED adminusers.c alloc.c authenticator.c atomic.c buffer.c config.c config_runtime.c dcb.c filter.c externcmd.c gwbitmask.c gwdirs.c hashtable.c hint.c housekeeper.c load_utils.c log_manager.cc maxscale_pcre2.c memlog.c misc.c mlist.c modutil.c monitor.c queuemanager.c query_classifier.c poll.c random_jkiss.c resultset.c secrets.c server.c service.c session.c spinlock.c thread.c users.c utils.c skygw_utils.cc statistics.c listener.c gw_ssl.c mysql_utils.c mysql_binlog.c modulecmd.c )
+add_library(maxscale-common SHARED adminusers.c alloc.c authenticator.c atomic.c buffer.c config.c config_runtime.c dcb.c filter.c filter.cc externcmd.c gwbitmask.c gwdirs.c hashtable.c hint.c housekeeper.c load_utils.c log_manager.cc maxscale_pcre2.c memlog.c misc.c mlist.c modutil.c monitor.c queuemanager.c query_classifier.c poll.c random_jkiss.c resultset.c secrets.c server.c service.c session.c spinlock.c thread.c users.c utils.c skygw_utils.cc statistics.c listener.c gw_ssl.c mysql_utils.c mysql_binlog.c modulecmd.c )
 
 target_link_libraries(maxscale-common ${MARIADB_CONNECTOR_LIBRARIES} ${LZMA_LINK_FLAGS} ${PCRE2_LIBRARIES} ${CURL_LIBRARIES} ssl pthread crypt dl crypto inih z rt m stdc++)
 

server/core/filter.cc

@@ -0,0 +1,61 @@
+/*
+ * 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/bsl.
+ *
+ * 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 <maxscale/filter.hh>
+
+namespace maxscale
+{
+
+//
+// FilterSession
+//
+
+FilterSession::FilterSession(SESSION* pSession)
+    : m_pSession(pSession)
+{
+}
+
+FilterSession::~FilterSession()
+{
+}
+
+void FilterSession::close()
+{
+}
+
+void FilterSession::setDownstream(const Downstream& down)
+{
+    m_down = down;
+}
+
+void FilterSession::setUpstream(const Upstream& up)
+{
+    m_up = up;
+}
+
+int FilterSession::routeQuery(GWBUF* pPacket)
+{
+    return m_down.routeQuery(pPacket);
+}
+
+int FilterSession::clientReply(GWBUF* pPacket)
+{
+    return m_up.clientReply(pPacket);
+}
+
+void FilterSession::diagnostics(DCB* pDcb)
+{
+}
+
+}
+