Consider storing events as pickled binary blobs in the DB

This issue has been migrated from [#12959](https://github.com/matrix-org/synapse/issues/12959).

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Folklore has it that Synapse spends a lot of time parsing json from text columns in the DB. It might be quicker to store and load a pickled version of the event. The event content is already an opaque blob AFAIK; I'm not sure we'd lose much by making an opaque binary blob rather than an opaque text blob. The first step would be to make some kind of vaguely representative benchmark and see how long depickling takes compared to parsing.