BUG/MEDIUM: sink: properly init applet under sft lock

BUG/MEDIUM: sink: properly init applet under sft lock

Since 09d69eacf8 ("MEDIUM: sink: start applets asynchronously") the applet
is no longer initialized under the sft lock while it was the case before.

At first it doesn't seem to be an issue, but if we look closer at
sink_forward_session_init(), we can see that sft->appctx is assigned
while it can be accessed at the same time from sink_init_forward().

Let's restore the old guarantees by performing the .init under the sft
lock.

No backport needed unless 09d69eacf8 is.