[CSL-1533] Add MonadMask-based versions of some MVar functions
[CSL-1533] Add MonadMask-based versions of some MVar functions

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype