Improve migration consistency check to reflect storage unit maxPoolSize dynamically

[sandynz]

Changes proposed in this pull request:

• Improve migration consistency check to reflect storage unit maxPoolSize dynamically

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[sandynz]

Usage:
Migration consistency check will build ShardingSphereDataSource with data source configuration from migration job configuration, including maxPoolSize data source property.
If maxPoolSize is not enough for consistency check (there's get connection error message in SHOW MIGRATION CHECK STATUS ...), then we can update maxPoolSize via ALTER STORAGE UNIT .... And restart consistency check job to make maxPoolSize take effect.