Introduce a channel name to ID cache

[timja]

Fixes #964

Testing done

Tested on a large workspace (~1900 channels) where this only worked for 1 file upload before it got rate limited before.
It now works as long as you don't clear the cache and then immediately try upload a file.

I added in some functionality to refresh when a channel isn't found so that new channels are picked up immediately, to de- risk it I made it not invalidate the old cache on errors.

I've tested it quite thoroughly, but its quite painful, really annoying of slack to implement it like this...

Submitter checklist

Preview Give feedback

1. Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[sebaturen]

Hi! This update removes the possibility to upload files under the threads using threadId from the last message sent. Is this intentional, or am I missing the option to use threadId again?

[timja]

Hi please create an issue with steps to reproduce.

I am able to upload files in threads using the instructions in the README