MEDIUM: streams: do not use the streams lock anymore

MEDIUM: streams: do not use the streams lock anymore

The lock was still used exclusively to deal with the concurrency between
the "show sess" release handler and a stream_new() or stream_free() on
another thread. All other accesses made by "show sess" are already done
under thread isolation. The release handler only requires to unlink its
node when stopping in the middle of a dump (error, timeout etc). Let's
just isolate the thread to deal with this case so that it's compatible
with the dump conditions, and remove all remaining locking on the streams.

This effectively kills the streams lock. The measured gain here is around
1.6% with 4 threads (374krps -> 380k).