Avoid allocs in dtors in futures collect
Avoid allocs in dtors in futures collect

Summary:
[Folly] Avoid allocs in dtors in futures `collect`.

`CollectContext`, a detail helper type, allocates storage for a results vector in its dtor. This is an awkward situation and should be avoided.

Reviewed By: ericniebler

Differential Revision: D6649299

fbshipit-source-id: 87746fcc78fa080f63505d7bb864aca6c4a3d7cb

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