fix: Increase default CONCURRENCY for replicator

[sds]

Motivation

Since most users seem to connect to a remote hub, increase the default concurrency.

Change Summary

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PR-Codex overview

Detailed summary

• Added a comment explaining how to set the CONCURRENCY variable in the .env file.
• Increased the value of the CONCURRENCY variable by multiplying it by 4 times the number of available processors.
• Added a note about replication messages appearing out of order and the reason for it.
• Added a note about the number of messages in the Postgres table not matching the number in the hub due to the replicator not backfilling "Remove" messages.
• Added information about being able to query data from Postgres while the syncing process is ongoing.

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Name	Status	Preview	Comments	Updated (UTC)
hub-monorepo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 31, 2023 9:01pm

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Latest commit: 4cb4e65

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