Improve NNI manager logging

[liuzhe-lz]

This PR implements following improvements:

1. Allow TS loggers to have "name" like Python counterpart, so it can distinguish log messages from different modules. This will be helpful for 3rd party training service.
2. Correctly format log message to string (rather than array).
3. Automatically stringify JSON objects and arrays, so you won't see [Object object] in log.

Before:

[2021-04-21 10:37:34] INFO [ 'Datastore initialization done' ]

Now:

[2021-05-10 12:07:56] INFO (root) Datastore initialization done