make wait() and friends return reference to self instead of new Future

make wait() and friends return reference to self instead of new Future

Summary:
we still make the new Future, but assign it to ourselves. this avoids the following buggy pattern that people might expect to work

```
auto f = ...
f.wait();

// Careful. f.value() was moved out into the new Future, so you may have lost something
someOperationOn(f.value());

// Nope. We already set a callback internally in wait()
f.then(...);
```

Test Plan: unit

Reviewed By: davejwatson@fb.com

Subscribers: exa, yfeldblum, trunkagent, fbcode-common-diffs@, sammerat, cold-storage-diffs@, folly-diffs@, jsedgwick, aflock

FB internal diff: D1809040

Tasks: 6048284

Signature: t1:1809040:1422900812:1b416408eb5eaa71e88778c9c22ed8bfba087efe