Auto-tune cache sizes

[ara4n]

Rather than expecting people to manually tune their cache sizes, surely Synapse should be able to figure it out itself (perhaps within some bounds), a bit like a JVM

[richvdh]

related: #2123

[richvdh]

also #6400

[erikjohnston]

https://dgraph.io/blog/post/introducing-ristretto-high-perf-go-cache/ has interesting ideas we could steal

[erikjohnston]

Specifically: it'd be a good start to implement TinyLFU to decide what we allow to be added to the cache. Not sure there are any python libraries for it, but there are Rust ones we might be able to use.