Allow discarding of data if we don't need it

Some systems simply want to look at attchments
and mail body, and don't actually queue files
to disk. For these systems we can simply discard
the data lines after the mailbody/mailheader
has parsed them.
Document discard_data
Fsync when writing outbound files
Merge branch 'master' into outbound_queue_async

Conflicts:
	outbound.js