[CSL-1578] Use mapConcurrently for launching plugins
[CSL-1578] Use mapConcurrently for launching plugins

The Mockable.Concurrent doesn't provide mapConcurrently_ but that would
be a little nicer.

The runWallet function waits for any of the plugins to fail. If
runWallet is killed, then those child threads will be killed
automatically too.

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