vconn-stream: Make definitions of tcp_vconn_class, etc., easier to find.

vconn-stream: Make definitions of tcp_vconn_class, etc., easier to find.

When object definitions are entirely hidden inside macros with token
pasting tricks, tools like "tags" and even "grep" cannot find them.  This
change makes the definitions of tcp_vconn_class and other stream-based
vconns easier to find, by exposing their definitions to these tools.

Reported-by: Yongqiang Liu <liuyq7809@gmail.com>