enhance(slack): improve startup speed

[wass3r]

on slack workspaces with large user counts the startup time can be quite long (witnessed 15m+) due to fetching all users, usergroups, and rooms.

the changes in this PR include:

• remove fetching users as it's currently not utilized - this was also the bulk of the problem
• make fetching usergroups and rooms async

i believe the original intent was to fetch once and have a local lookup and only turn to Slack api calls when needed. it might be re-introduced at a later point in a sensible manner.

[coveralls]

Pull Request Test Coverage Report for Build 5308635091

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 5157599379:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls