re-create them immediately after a trace sends its last message.
Store them, one per TraceId, in ArenaStruct. Cope correctly if they
cannot be created (ControlAlloc fails). Destroy them correctly on
mps_arena_destroy. See job001989 and design/message-gc#lifecycle
(not yet written).
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is not essential to Trace objects. (p4 integ'd from trace.c to
preserve ancestry).
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Change: 166900
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