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/modules/filter/test/mock_session.cc

@@ -37,16 +37,11 @@ Session::Session(Client* pClient)
 
     strcpy(m_mysql_session.db, "dummy");
 
-    pSession->variables = new SessionVarsByName;
-
     m_client_dcb.data = &m_mysql_session;
 }
 
 Session::~Session()
 {
-    delete variables;
-    delete last_statements;
-    delete dcb_set;
 }
 
 Client& Session::client() const