SESSION_CONNECT_ATTRS 

Before this fix, executing a select on table
performance_schema.session_connect_attrs could crash the server,
when executed under load.

The root cause is that the code that scans records:
- does not honor the PFS_thread::m_sesion_lock lock,
- does parse the live session attribute data,
  while it can possibly be modified

Race conditions could occur:
- when writing session attributes, shortly after a thread connect
- when erasing session attributes, shortly before a thread disconnect

The fix is to:
- use an optimist lock on PFS_thread::m_sesion_lock
- make a copy of the session connect attributes,
- only when the optimist lock was successful,
  (meaning the data was not concurently modified),
  parse the safe copy of the content of it to extract attributes.