Fix migration might skip some records on big table after job restarting

[sandynz]

Related to #33996.

Changes proposed in this pull request:

• Remove InventoryDumperContext.firstDump

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[sandynz]

InventoryDumperContext.firstDump might cause bug.

Test case

Table and configuration:

• tableA with 300 million records, with integer primary key id AUTO_INCREMENT. Use default SHARDING_SIZE 10 million, then there could be 30 records shards.
• Use default WORKER_THREAD 20.

Steps to reproduce

1. Start migration job. (There will be 20 threads to do migration concurrently, and 10 shards tasks are queued)
2. After job running a very short time, run show migration status {jobId};, make sure processed_records_count is greater than 0 and inventory_finished_percentage is less than 10%.
3. Restart migration job.

Reason analyze

When job is stopped and there are still some shards tasks are queued, after job is started again, InventoryDumperContext.firstDump will return false (check on job item level, not on every shard task). When queued shards tasks running, query SQL condition will use >, so a shard task will skip one record, >= should be used at the first dump for every shard task.