MINOR: trace: move the known trace context into a dedicated struct

MINOR: trace: move the known trace context into a dedicated struct

We now have a trace_ctx to hold the sess, conn, qc, stream and so on.
This will allow us to pass it across layers so that other helpers can
help fill them.

Ideally it should be passed as an argument to __trace_enabled() by
__trace() so that it can be passed back to the trace callback. But
it seems that trace callbacks are smart enough to figure all their
info when they need them.