Fix crash on trx replay with session command

Readwritesplit would crash with the following transaction:

    BEGIN;
    SET @a = 1; -- This is where it would crash
    COMMIT;

When a session command was a part of the transaction, empty queries
(i.e. NULL GWBUFs) would be added to the transaction. If the transaction
were to be replayed, MaxScale would crash when these NULL queries were
executed.

Once the empty responses were fixed, the replaying of the transaction
would fail with a checksum mismatch. This was caused by the wrong order of
processing in RWSplitSession::clientReply. The response processing for
session commands was done after the response processing for replayed
transactions. This would trigger a checksum comparison too early for the
transaction in question.

server/modules/routing/readwritesplit/trx.hh

@@ -18,6 +18,7 @@
 
 #include <maxscale/buffer.hh>
 #include <maxscale/utils.hh>
+#include <maxscale/modutil.hh>
 
 // A transaction
 class Trx
@@ -38,6 +39,13 @@ public:
      */
     void add_stmt(GWBUF* buf)
     {
+        ss_info_dassert(buf, "Trx::add_stmt: Buffer must not be empty");
+
+        if (MXS_LOG_PRIORITY_IS_ENABLED(LOG_INFO))
+        {
+            MXS_INFO("Adding to trx: %s", mxs::extract_sql(buf, 512).c_str());
+        }
+
         m_size += gwbuf_length(buf);
         m_log.emplace_back(buf);
     }
@@ -123,6 +131,7 @@ public:
      */
     void close()
     {
+        MXS_INFO("Transaction is complete");
         m_checksum.reset();
         m_log.clear();
         m_size = 0;