Changing mailbox parameter from native pointer to smartpointer

The SmartPointer is needed as due to multi threading it is not know
who should delete the mailbox at the end. Either the generated code needs
to delete it or the stack itself. This problem is solved by using
smartpointers.

Change-Id: I2c1ac6ca3ed126afa438e700f5849ffdaf03b2bb

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1578873 13f79535-47bb-0310-9956-ffa450edef68