fix: skipped SSR cache usage & precached data return

[fabis94]

Two bugs:

• In SSR the fetchPolicy is always forced to be network-only? This makes no sense - what if you invoke the same exact operation from multiple places in the app? network-only is gonna skip cache and re-do the same exact request multiple times. Definitely not good, kinda crazy that this has been in the lib for so long.
• In SSR, waiting for results relies on onServerPrefetch, which is OK enough, but not enough for non-component use cases (e.g. Nuxt plugins/middlewares). Instead of just relying on onServerPrefetch, I made it also re-use the same logic CSR uses where if there's cached data already, it returns it immediately.

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commit: 148d337