Add support for query requesting information about partition step distribution across workers

[jtfmumm]

You can now send an external message to a running
Wallaroo cluster and receive a response with a json
payload containing the location of all steps (by id)
in each partition (state and stateless) across all
workers.

This commit also includes an update to external_sender
adding the ability to send partition queries and print
the response to stdout.

Closes #2024

[jtfmumm]

To test this, start up any application with a partition (word count is best because it has both state and stateless partitions). Run it with multiple workers and/or grow the cluster. Use the external_sender tool with the following command (provided you're listening on an external channel of 127.0.0.1:5050):

./external_sender -e 127.0.0.1:5050 -t partition-query

You should see a json map printed out to stdout with the following structure:

{partition_type: {partition_id: {worker: step_ids}}}

You can query after grow events to see that it's updating (and doing so, you'll quickly discover #2026 as I did).

[SeanTAllen]

@john this seems like something that should have some unit tests.

[jtfmumm]

@SeanTAllen There are unit tests around the actual json encoding/decoding. Which part do you mean?

[SeanTAllen]

Sorry I completely missed those.