Trigger redis to create a snapshot

1. The client sends a BGSAVE command to redis, and a subprocess is created to write the snapshot to storage, and redis can receive commands normally.

2. The client sends a SAVE command to redis, and redis does not receive the command before executing the write storage.

3. Snapshots of configuration file settings, such as save 60 5000, can be configured multiple. When the condition is met, the BGSAVE command is triggered.

4. When SHUTDOWN, SAVE command will be triggered.

5. After slave redis sends off the SYNC command, the master starts the BGSAVE command.