Backport to 0.1: Shared must relinquish control to the executor if repolled #2154
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This backports #2136 to Futures 0.1. There isn't much to add on top of
what's mentioned in #2136: the same bug exists in Futures 0.1, and it'll
fail in the same way when polled in recent versions of Tokio (#2418).
Backporting to 0.1 allows codebases that still have some Futures 0.1
code around to still use newer versions of Tokio.