Refactor ClientSocketContext write(2) using Server:: write methods

Refactor ClientSocketContext write(2) using Server:: write methods

Writing to the client connection is scoped as an action for class Server
and its child classes. There is no need for ClientSocketContext to be
providing the callback handlers and performing I/O error handling.

With Server providing the current write handler we can move from
CBDATA callbacks to AsyncCall. Initial testing indicates this has some
minor performance benefit.