Better connection management strategy with netty support

[bruceleexiaokan]

The current connection management behavior looks like always trying to find an idle connection for a new request. If there's no idle connection and the max connection threshold reaches, the client will try to create a new one instead of reuse existing one. It will cause problem of fast connections & disconnections, especially for TLS connection, the cost of connection is very high. We need a different connection management, which can reuse the existing connection even the response may not be returned. Netty can easily support it, but need to enhance the client logic to support the new connection management strategy.

[BrianNichols]

Sharing a connection between multiple concurrent transactions is on our road map, but it requires protocol changes that affects the server. When the server team gets some free time, this project will be addressed.