API 1.0 - Remove the cache layer

[BrynCooke]

Decision below is to remove the cache layer

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Original body:

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What layers do we want to expose to plugin authors? We currently have:

• checkpoint
• instrument
• cache
• map_future_with_context (what an awful name)

Do we need higher level constructs? Revisit what we have.

[garypen]

Re-examine requirements for ServiceBuilderExt and ServiceExt and ensure that if both are still required they are consistent and everything is well-named.

[garypen]

Should we remove:

• cache ?

[abernix]

We agreed together we should delete the cache.

[abernix]

@SimonSapin @BrynCooke I moved this from "Needs design" to "Todo", but let me know if you feel differently — not sure if the summary needs refinement, but feel free to edit.