Restore feature: wait for running handlers
Restore feature: wait for running handlers

Oops! This was lost in a refactor.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,serokell/tw-rework-sketch,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl