Changes:

• see notes on release v0.21.0-alpha.1;
• see notes on release v0.21.0-alpha.2;
• see notes on release v0.21.0-alpha.4;
• the tokio crate version was updated to 0.2;
• the bytes crate version was updated to 0.5.2;
• the mysql_common crate version was updated to 0.20.0;