XD-2047 Expose Redis Bus Headers Customization

JIRA: https://jira.spring.io/browse/XD-2047

The redis bus supports passing custom (string-valued) headers but
there was no easy way to configure them.

Add a property to `application.yml`, `servers.yml` to allow
customization.
XD-2015 Fix UI end-to-end (E2E) tests

* Made the documentation (README.md) clearer regarding issue `Could not find chromedriver...`
* Due to the recently introduced addition of polling the E2E tests were partially broken also.
  Add `browser.ignoreSynchronization=true` to the tests as described at https://github.com/angular/protractor/issues/49

XD-2015 Reference `protractor.conf.js` explicitly

* Bump versions in `package.json`
* Have NPM execute `webdriver-manager update`

XD-2015 Polishing, updating `README.md`