Set finalResponsePromise before calling continueExhaustRequest in NetworkInterfaceTL

See comment:3 on BF-19523 for a more detailed description of this problem; there is a small window of time where `continueExhaustRequest` can try to assign a promise to the `finalResponsePromise` before the `finalResponsePromise` has been set to a non-null promise. Move the assignment of `finalResponsePromise` to before the asynchronous call to `continueExhaustRequest`.