In mysql_execute_command(), move optimizer trace initialization to be
after run_set_statement_if_requested() call.

Unfortunately, mysql_execute_command() code uses "goto error" a lot, and
this means optimizer trace code cannot use RAII objects. Work this around
by:
- Make Opt_trace_start a non-RAII object, add init() method.
- Move the code that writes the top-level object and array into
  Opt_trace_start::init().