rerun rrd compact: always put blueprints at the start of the recordings

[teh-cmc]

Slight modification to rerun rrd compact in order to make sure that, if the recording contains one or more blueprints, they end up at the beginning of it, otherwise the experience would suck since the blueprint would only get loaded once all the data had finished streaming in.

• DNM: requires Add Transformables subscriber for improved TransformContext perf #6997

cc @jleibs

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