trace: rework alloc tracing

trace: rework alloc tracing

Remove trace, we use debug log for that
Make alloc trace simpler, removing some methods.
Activate alloc trace with a GST_TRACE=3 environment variable.
Dump leaked objects atexit.
Provide an offset in the object where the GType can be found so that more
verbose info can be given for objects.
Remove -T option from gst-launch because tracing is now triggered with the
environment variable.