Add atomically_replaced_file for safer output

Use `tempfile.mkstemp` to write files safely and then rename them
atomically to the target filename at the end. This guarantees that two
concurrent processes trying to write the same target file will result
in one process "winning" with a consistent version.

Use this to protect:
 * midx writing
 * .idx cache writing

Previously both of these processes `open(destination + '.tmp', 'w')`
which would lead to concurrent processes writing the same tmp file at
the same time.

See also https://groups.google.com/d/msg/bup-list/qdPUaXGO1cI/g8K2aFaaXz0J

Signed-off-by: Nathan Bird <ecthellion@gmail.com>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
Adjust atomically_replaced_file documentation.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
thelpers.py: use t/tmp as the tmpdir parent

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Merge work on atomic writes