Clean up TR_MSG, hopefully getting talloc context handling right

Clean up TR_MSG, hopefully getting talloc context handling right

I had assumed in a few places that TR_MSGs and the various message
payload types were always allocated dynamically via talloc(). This is
not a safe assumption - in a few places, we use stack-allocated TR_MSGs
and these are all used outside our code via the libtr_tid library.
We now use talloc when we can (i.e., when we have encoded or decoded
a message and know we used talloc), but otherwise leave it to the calling
code to properly manage memory.