Delay node: add option to send intermediate messages on separate output

[knolleary]

• New feature (non-breaking change which adds functionality)

Proposed changes

Backlog item: https://trello.com/c/1jAoRESp

This changes the 'drop intermediate messages' checkbox of the Delay node (in rate limit mode) to a select box. The options are:

• Queue intermediate messages - the default behaviour, equivalent to the old checkbox being unticked
• Drop intermediate messages - equiv to the old checkbox being ticked
• Send intermediate messages on 2nd output.

If this last option is selected, the node gains a 2nd output. Any message that would previously have been dropped will now get sent on that output.

This is useful for rate limiting flows that need to do some work with the 'reject' messages. For example, rate limiting an HTTP API, where the 'dropped' requests need to be responded to with a 503.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.01%) to 67.681% when pulling 1c18641 on output-discarded-rate-limit into 1419729 on dev.