Move client back to using MVFST by default.
Move client back to using MVFST by default.

Summary: After the push is complete the next push we should be able to use MVFST again.

Reviewed By: yangchi, lnicco

Differential Revision: D16226842

fbshipit-source-id: 9737b37e94f6834813ed6a2c8825382a968cbfaa

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst