MINOR: trace: retrieve useful pointers and enforce lock-on

MINOR: trace: retrieve useful pointers and enforce lock-on

Now we try to find frontend, listener, backend, server, connection,
session, stream, from the presented argument of type connection,
stream or session. Various combinations and bounces allow to
retrieve most of them almost all the time. The extraction is
performed early so that we'll be able to apply filters later.

The lock-on is set if it was not there while the trace is running and
a valid pointer is available. If it was already set and doesn't match,
no trace is produced.