Abstract unified connection layer

# 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.