notmuch dump: Free each message as it's used.

Previously we were leaking[*] memory in that the memory footprint of
a "notmuch dump" run would continue to grow until the output was
complete, and then finally all the memory would be freed.

Now, the memory footprint is small and constant, O(1) rather than
O(n) in the number of messages.

[*] Not leaking in a valgrind sense---every byte was still carefully
being accounted for and freed eventually.