Add Support for SQL Database Storage in CurveBS

CurveBS currently use etcd as metadata storage but etcd has limited scalability, and the amount of metadata that can be stored is limited.So i plan to add a MySQL database as one of the storage engines and let user choose whether the metadata is stored in the kv engine or the sql engine through the configuration file. Deliverables: 1.A new SQL storage client implementation for metadata in CurveBS. 2.Configuration options to choose between kv engine and SQL engine for metadata storage. 3.Modifications to the leader election process to support the new SQL storage client. 4.Documentation on how to configure and use the new SQL storage client. 5.Tests to ensure the new SQL storage client works as expected and is compatible with existing functionality. 6.Release notes detailing the changes and improvements made to CurveBS with the addition of the new SQL storage client.