MXS-1929: Add Session class

The Session class now contains all of the C++ objects that were previously
in the MXS_SESSION struct. It is also allocated with new but all
initialization is still done outside of the Session in session_alloc_body.

This commit will not compile as it is a part of a set of commits that make
parts of the session private.

server/core/internal/session.hh

@@ -13,6 +13,13 @@
  */
 
 #include <maxscale/cppdefs.hh>
+
+#include <deque>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
 #include <maxscale/session.h>
 #include <maxscale/resultset.hh>
 #include <maxscale/utils.hh>
@@ -40,4 +47,22 @@ struct RegistryTraits<MXS_SESSION>
 
 }
 
+typedef struct SESSION_VARIABLE
+{
+    session_variable_handler_t handler;
+    void*                      context;
+} SESSION_VARIABLE;
+
+typedef std::unordered_map<std::string, SESSION_VARIABLE> SessionVarsByName;
+typedef std::deque<std::vector<uint8_t>> SessionStmtQueue;
+typedef std::unordered_set<DCB*> DCBSet;
+
+class Session: public MXS_SESSION
+{
+private:
+    SessionVarsByName m_variables;
+    SessionStmtQueue  m_last_statements; /*< The N last statements by the client */
+    DCBSet            m_dcb_set;         /*< Set of associated backend DCBs */
+};
+
 std::unique_ptr<ResultSet> sessionGetList();