out_forward: Make forwardable for msgpack payloads of ctraces and cmetrics
out_forward: Make forwardable for msgpack payloads of ctraces and cmetrics

Just using to pack `msgpack_pack_bin` is not suitable for them.
Instead, we should format as forward mode compatible array with:

```
[time, [{map of msgpack payloads}]]
```

to be able to handle with ordinary msgpack handler for format of forward mode.

Signed-off-by: Hiroshi Hatake <a476a6e5b5893650242fbf0b2961500a9bd9032e@calyptia.com>

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