SSE Ext #2225: reinstantiate EventSource listeners upon reconnection logic

[vlad-tkachenko]

Description

Please describe what changes you made, and why you feel they are necessary. Make sure to include
code examples, where applicable.

Currently when EventSource loses connection with the server, e.g. when server restarts and later SSE ext reconnects it creates new EventSource, but is not adding any previously registered listeners. As a result while server issues new events, SSE ext no longer consumes them.

Corresponding issue:

Refs: #2225

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

Tested on a real application by using the exact changes and patching the npm package with patch-package module first.
All the existing tests are passing:

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

Last isn't checked, as it fails to start on my machine even on a clean dev branch.

[Renerick]

Hello and thank you for the contribution! Your PR highlighted an issue with the whole registerSSE logic, so I took the liberty to modify it a bit more and update this PR. Your change LGTM, but I would like to also ask you to check out my changes and verify that everything still works as expected with regards to reconnection logic and everything else. Thanks!

[vlad-tkachenko]

Hi @Renerick,

Made a quick test, no issues found, reconnection logic works as intended.
Cheers.