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