MINOR: stconn: Report a send activity when endpoint is willing to consume data

MINOR: stconn: Report a send activity when endpoint is willing to consume data

When the endpoint (applet or mux) is now willing to consume data while it
said it wouldn't, a send activity is reported. Indeed, the writes was
blocked because of the endpoint. It is now ready to consume outgoing
data. So an send activity must be reported to reset corresponding timers.

Concretly, when the flag SE_FL_WONT_CONSULE is removed, a send activity is
reported.