Make the CLI thread-safe

The CLI is not currently thread safe. In most cases this isn't a big deal because the CLI is wrong from a command line and so each instance of daffodil runs in its own process space and we don't have to worry about thread safety.

However, when testing the CLI, we run it in separate threads instead of forking, which substantially decreases the time needed to run tests. But because the CLI isn't thread safe, we must disable parallel execution of tests in daffodil-cli/build.sbt.

Ideally we could modify the CLI so it was thread safe, which would allow parallel test execution. It would also also libraries to run the CLI without forking if it was something they needed.