Updates to test-client concurrent-connections command

[michaelbarnes]

This PR introduces small updates to the test-client concurrent-connections command, based on feedback from a customer who's using the test-client to perform load-testing. Here's what's changed:

1. The logging output from the concurrent-connections command has been made the same regardless of the mode param provided i.e. Websocket and HTTP mode now print the same types of messages e.g. checkpoint_complete op_id: ${last_op_id}, ops: ${numOperations}, bytes: ${size}, duration: ${duration}ms, data: [${printData}].
2. A new print (-p or --print) param was added to the concurrent-connections command. Developers can provide a field that displays the corresponding values from the synced source database, once the sync has completed. The id value will print by default if no argument is provided for the print command.
3. The README of the test-client has been updated with more details on each of the available commands.

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[rkistner]

The default behavior of printing the id can cause a lot of output to the console, especially when using this for a typical case of testing bulk initial sync. I'd recommend disabling that option by default, instead of defaulting to printing the id.

Otherwise this looks good to me.

[rkistner]

@michaelbarnes I rewrote the logic a bit:

1. The websocket/http implementations now only take care of the "transport-level" logic.
2. There is now only one implementation that parses the individual messages, built on top of the transport above.
3. Both http and websocket implementations now support automatic retries, resuming at the last synced bucket positions.
4. I added a "--once" option to stop after the first checkpoint.
5. The print option defaults to not printing anything.