[lib] Make sure we send only one join thread request at a time

Summary:
It sometimes happens that en effect fires multiple times. This protects us against sending multiple requests to the server.

https://linear.app/comm/issue/ENG-8958/invite-invalid-when-trying-to-join-the-comm-community

Depends on D12999

Test Plan: Checked if it is still possible to join a community using invite links. Checked if accepting two invites in a row works.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13000