body-to-be-discarded flags on disk

[Uday Reddy]

The theory is that if a message has its body-to-be-discarded flag set, then its body should be discarded before the folder is written to disk.

If that is done consistently, then when we read a folder from the disk, no message should have the body-to-be-discarded flag set. But we are finding messages that have them set.

This should be cleaned up. Once that is done, we can revert the body-to-be-discarded flag position to the headers-to-be-retrieved flag, which was the original owner of this position. Heaven won't fall if we don't return the position, but it would be cleaner to do so.