pipeline: pipeline is blocked on macos

[feely]

If run the mnist pipeline on mac, it will be blocked at:

const tf = boa.import('tensorflow');

[feely]

Costa runs plugins by 2 steps:

1. load plugin source by require.resolve('path/to/plugin')
2. call the entry function

In this case, the pipeline is blocked by step 1, so maybe we could add a monitor for job running, and the job running status could be plugin-loading, plugin-running, failed. If load timeout, the monitor could kill it and mark the job failed.