When missing intermediate and/or parent messages in a thread, the
printed thread tree branches were completely invalid. This commit makes
sure thread node entries that have no corresponding envelopes are
accounted for in the tree structure.
Instead of using Union/Find to gather mail that belongs in the same
e-mail thread together, add a new entity ThreadGroup that ThreadNodes
point to. ThreadGroup represents an actual Thread: A thread root
ThreadGroup::Group or a reply ThreadGroup::Node.
To make semantics more accurate:
- ThreadNode hash should be renamed to ThreadNodeHash
- ThreadGroupHash should be renamed to ThreadHash
- ThreadGroup::Group should be a struct named Thread instead
- move ThreadGroup::Node logic to ThreadNode akin to Union/Find
- rename ThreaddGroup::Group to Thread