[lib] Batch BaseAutoJoinCommunityHandler

Summary: This diff updates BaseAutoJoinCommunityHandler to work in batches of 5, and to avoid triggering a joinCommunity for a single community more than once.

Test Plan: Varun tested this in his local environment, where he has set up a reproduction of the production environment where we have a lot of communities tagged with Farcaster channels, and he tested signing in as a user with a lot of channels

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13583