Revert "Update to stables futures"

[rphmeier]

Reverts #81

[rphmeier]

@tomaka I bumped your PR over to the v0.11.0 branch to be reapplied later. I need to issue a v0.10.0 soon with changes for Substrate.

[codecov-io]

Codecov Report

Merging #89 into master will decrease coverage by 0.07%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   88.54%   88.47%   -0.08%     
==========================================
  Files          10       10              
  Lines        3117     3114       -3     
==========================================
- Hits         2760     2755       -5     
- Misses        357      359       +2

Impacted Files	Coverage Δ
src/testing.rs	93.24% <100%> (ø)	⬆️
src/bridge_state.rs	96.29% <88.88%> (ø)	⬆️
src/voter/voting_round.rs	81.2% <95.83%> (-0.05%)	⬇️
src/voter/past_rounds.rs	93.33% <96.96%> (-1.3%)	⬇️
src/voter/mod.rs	82.77% <97.27%> (+0.09%)	⬆️

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