Add --chunk-concurrent-size for parallel row-copy

[dnovitski]

Summary

Port of #1398 by @shaohk to current master, with correctness improvements.

Adds --chunk-concurrent-size flag that allows multiple row-copy chunks to execute in parallel within each iteration using errgroup. Default is 1 (no behavior change).

Motivation

On large tables with fast storage (NVMe/SSD), the single-threaded row-copy loop can become a bottleneck. This flag enables parallel chunk copying to improve migration throughput.

Performance Results

1M rows, ADD COLUMN extra_col INT DEFAULT 0, Docker MySQL 8.0, chunk-size=1000:

Concurrency	Wall-clock	Speedup
1 (default)	30.6s	baseline
4	23.9s	22% faster
8	20.9s	32% faster

Benefits scale with table size and storage throughput.

Key Design Decisions

• Two execution paths: concurrency=1 matches master's retry semantics exactly (range calc inside retry loop for hook-based chunk size reduction); concurrency>1 pre-calculates ranges under mutex for safe parallel execution
• Thread-safe range calculation: CalculateNextIterationRangeEndValues(advanceCursor bool) protected by mutex, returns *IterationRangeValues struct with isolated Min/Max per goroutine
• No shared mutable state in hot path: SQL warnings returned as function value (eliminates data race on shared MigrationLastInsertSQLWarnings field)
• errgroup with real migration context: Proper cancellation propagation when any chunk fails
• DB pool auto-sizing: Connection pool increased when --chunk-concurrent-size exceeds default pool size
• Backward compatible: Default concurrency=1 preserves existing single-threaded behavior exactly

Changes from original #1398

• Adapted to current master API (builder pattern, receiver names, retryBatchCopyWithHooks)
• Fixed thread-safety: ApplyIterationInsertQuery returns SQL warnings instead of writing to shared field
• Correct retry behavior: single-threaded path recalculates range on retry (matches master); concurrent path retries same range (INSERT IGNORE is idempotent)
• Proper IncludeMinValues handling for first iteration
• Uses real migration context (not context.Background())

Testing

• All CI checks pass (build, lint, CodeQL, migration tests on MySQL 5.7/8.0/8.4/Percona)
• Performance benchmarked (22-32% improvement with concurrency 4-8)
• Data integrity verified (1M rows, checksum match)
• TestRetryBatchCopyWithHooks passes (hook-based chunk size reduction works correctly)

Checklist

• Tests pass
• Documentation updated (doc/command-line-flags.md)
• Backward compatible (default=1)

Based on work by @shaohk in #1398.