chain/migrations refactor: optionally set max migration workers

[frrist]

Proposed Changes

Allow the maximum number of workers for state migrations to be specified via an environment variable LOTUS_MIGRATION_MAX_WORKER_COUNT

Additional Info

Limits the number of actors being migrated simultaneously, thus reducing the overall memory footprint required by migrations. Motivated by experience in production where nodes with high CPU count exhaust all available memory due to a high number of workers loading actor states into memory to perform migration.

Checklist

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