A big refactory for the project

[kerryjiang]

The changes include:

• SuperSocket.Channel was renamed to SuperSocket.Connection. The related classes' names were also changed, like PipeChannel was renamed to PipeConnection.
• Improved the design of ConnectionListener, ConnectionBuilder and related classes.
• Added a new project SuperSocket.Server.Abstractions and move some interfaces and basic classes of server side into it. With this new project, the server side extension projects don't have to depend on the project SuperSocket.Server.
• ...

[kerryjiang]

Did it using git command.