Rename _MassageProcData to _JoinProcDataPerMinor

That's what the method actually does, so there is no reason why we
should use a funny but unreadable name.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>