Share data between identical session commands

If two or more session commands contain identical buffers, the buffer of
the first session command is shared between the others. This reduces the
amount of memory used to store repeated executions of session commands.

The purging of session command history in readwritesplit was replaced with
session command de-duplication. This was done to prevent problems that
could arise when the order of session commands plays a significant role.

server/core/session_command.cc

@@ -92,4 +92,11 @@ std::string SessionCommand::to_string()
     return str;
 }
 
+void SessionCommand::mark_as_duplicate(const SessionCommand& rhs)
+{
+    ss_dassert(eq(rhs));
+    // The commands now share the mxs::Buffer that contains the actual command
+    m_buffer = rhs.m_buffer;
+}
+
 }