Caching plugins for state modules

[aleclarson]

When a page is rendered, it produces its page-specific state module and any non-inlined state modules. These state modules need to be cached somewhere, so the server can respond to the inevitable requests for them. Currently, they are cached in-memory, but this only works for long-running servers. For serverless functions, a Redis instance or S3 could be used, but there's no easy way for plugins to control this.