Divide session.h to internal and interface headers

Definitions and function used by core are in
server/core/maxscale/session.h
Definitions and function used by plugins are in
include/maxscale/session.h

server/core/maxscale/session.h

@@ -0,0 +1,71 @@
+#pragma once
+/*
+ * 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.
+ */
+
+/**
+ * @file core/maxscale/session.h - The private session interface
+ */
+
+#include <maxscale/session.h>
+
+MXS_BEGIN_DECLS
+
+#define SESSION_STATS_INIT {0}
+#define MXS_DOWNSTREAM_INIT {0}
+#define MXS_UPSTREAM_INIT {0}
+#define SESSION_FILTER_INIT {0}
+
+#define SESSION_INIT {.ses_chk_top = CHK_NUM_SESSION, .ses_lock = SPINLOCK_INIT, \
+    .stats = SESSION_STATS_INIT, .head = MXS_DOWNSTREAM_INIT, .tail = MXS_UPSTREAM_INIT, \
+    .state = SESSION_STATE_ALLOC, .ses_chk_tail = CHK_NUM_SESSION}
+
+#define SESSION_PROTOCOL(x, type)       DCB_PROTOCOL((x)->client_dcb, type)
+
+/**
+ * Filter type for the sessionGetList call
+ */
+typedef enum
+{
+    SESSION_LIST_ALL,
+    SESSION_LIST_CONNECTION
+} SESSIONLISTFILTER;
+
+int session_isvalid(MXS_SESSION *);
+int session_reply(void *inst, void *session, GWBUF *data);
+char *session_state(mxs_session_state_t);
+bool session_link_dcb(MXS_SESSION *, struct dcb *);
+
+RESULTSET *sessionGetList(SESSIONLISTFILTER);
+
+void printAllSessions();
+void printSession(MXS_SESSION *);
+
+void dprintSessionList(DCB *pdcb);
+void dprintAllSessions(struct dcb *);
+void dprintSession(struct dcb *, MXS_SESSION *);
+void dListSessions(struct dcb *);
+
+/**
+ * @brief Get a session reference
+ *
+ * This creates an additional reference to a session which allows it to live
+ * as long as it is needed.
+ *
+ * @param session Session reference to get
+ * @return Reference to a MXS_SESSION
+ *
+ * @note The caller must free the session reference by calling session_put_ref
+ */
+MXS_SESSION* session_get_ref(MXS_SESSION *sessoin);
+
+MXS_END_DECLS