* invoke drainChannels prior to exiting from ManagedCmd::Wait()
* ManagedCmd timeout is just a special case of Stopping now to prevent goroutine leaks
* CommandChain was leaking goroutines on subcommand error. Chains now
  report a single aggregate error to prevent goroutine leaks.
Merge pull request #526 from mozilla-services/features/505-processinput-fixes

CommandChain and ManagedCmd fixes to prevent goroutine leakage