Abstract unified connection layer

[EarthChen]

Background

Currently dubbo does not abstract the connection very well.

A client is defined in the dubbo protocol to hold the connection.
A logical connection is defined in the Triple protocol to hold the connection.

The invoker gets the connection to execute the request.

For the multi-connection implementation, the dubbo protocol directly creates and holds multiple connections during initialization, and the triple protocol currently does not have a multi-connection implementation.

Describe the proposal

Hope to achieve the features

• Abstract unified connection layer implementation, support various protocol access
• Enhance connection-related management functions, provide connection pool, etc.

[conghuhu]

assign to me. Thanks

[lan-dian]

assign to me. Thanks.