refactor(BlockchainWriter): able to run as independent module

[krobi64]

PR Process - PR Review Checklist

Release

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Description of Changes

Resolves #868

1. Always run BatchWriter as if enableAnchoring is true
2. Add helpers for startup and shutdown
3. Add an index file to BlockchainWriter module
4. Add a script to package.json to run the index file
5. Add a Dockerfile with the specific command to run
6. Add a poet-node_blockchaIn-writer block to docker-compose.yml

TODO:

• [ ] Verify tests still run
• [ ] Determine a manual test
• [ ] See if there's a simple strategy to update tests to use the separate module.

[lautarodragan]

This is looking great!

[lautarodragan]

• Tested manually and it's working :rocket:
• Current test suite runs fine according to Travis :rocket: :rocket: