Refine r2/kv detach buffer flag to avoid copy

[jasnell]

Concerns over the possible performance hit of copying were raised. This adjusts the flag to preserve the post-call modification behavior when the detach flag is not set. Also sets the flag as explicitly experimental so we can experiment with it before releasing.

[a-robinson]

Hm, so did we just never fix the underlying concern here? I'm trying to find any sort of resolution but all I see is:

• Update R2/KJ to avoid mod-after-put when using ArrayBuffer #309 was merged with a fix
• Revert "Update R2/KJ to avoid mod-after-put when using ArrayBuffer" #314 reverted Update R2/KJ to avoid mod-after-put when using ArrayBuffer #309 and referenced this PR as a possible end-state
• This PR was closed a month after it was opened with no further comment

I don't see any clear conclusion either here or anywhere I could find internally. Do you remember where this ended up, @jasnell ?

[kentonv]

@a-robinson I believe nothing ever happened. There's some disagreement over whether we should do anything -- the existing behavior is technically wrong but we have no evidence that anyone has ever been impacted, whereas "fixing" it would involve a performance hit, at least in the absence of some sort of copy-on-write feature being added to V8.