Cache converted config objects

[The-Compiler]

With some config types (like UrlPattern), converting them is somewhat expensive. The cache in #4185 helps in places where this is really bad, but maybe we should do this for the config in general.

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Some more random thoughts from May 2020:

• If we have such a cache, it means we only need to call to_py when an object was changed (or we suspect it was, see Re-think mutable object handling for config #4344), which then also obsoletes Separate config validation from conversion #4192.
• Instead, we could use the to_py value as the "internal" one, but then we would need to have some way to convert from that to a YAML object again (i.e. the exact opposite of to_py). Not sure if that's possible (or how complex it is).