stop handling `then` when `next` is called

[twksos]

Pre-Submission Checklist

• [v] Opened an issue discussing these changes before opening the PR
• [v] Ran the linter and tests via make prepush
• [v] Included comprehensive and convincing tests for changes

Issues

Closes:

• Issue #1935

Summarize the issues that discussed these changes

currently, the async function handling does not meet user's expectation. a function returning Promise can still have the next param and:

• it could get called twice.
• if user send out an error through the next, the promise could still resolve.

Changes

add a local variable to check whether next is called and wrap the next function.

What does this PR do?

• in chain.js prevent further Promise handling if next is called.
• add tests

[twksos]

Hi @mmarchini, could you review this PR when you have time?