BUG/MEDIUM: stconn: Report read activity when a stream is attached to front SC

BUG/MEDIUM: stconn: Report read activity when a stream is attached to front SC

It only concerns the front SC. But it is important to report a read activity
when a stream is created and attached to the front SC, especially in TCP. In
HTTP, when this happens, the request was necessarily received. But in TCP,
the client may open a connection without sending anything. We must still
report a first read activity in this case to be able to properly report
client timeout.

This patch must be backported to 2.8.