Add Index on system_config.`key` to Accelerate Query

[wtune]

Missing index on table system_config column key might make the underlying query issued via ISystemConfigService#selectByKey slow. Since table system_config is mostly read and rarely updated, the performance impact for write operations is acceptable under common cases.

[atjiu]

The data in the system_config table is limited, and it is cached after being queried once in the SystemConfigService. Subsequent queries will not be queried in the database unless the data in the system_config table changes, so this index is the same whether it is added or not.