MINOR: config/trace: Add a 'traces' section to declare debug traces

MINOR: config/trace: Add a 'traces' section to declare debug traces

It is no longer supported to declare debug traces, via 'trace' directive, in
a global section. A 'traces' directive must be used instead. The syntax of
the 'trace' directive in these sections remains the same. But it is no
longer experimental.

The main reason for this change is to avoid to have a ring section defined
before a global one. Indeed, for now, forward declarations of ring sections
are not supported. So to configure traces, you had to add a ring section
before the global one defining the traces. Most of time, that meant to have
two global sections :

  global
    [...] # global settings

  ring <name>
    [...]

  global
    [...] # trace config

In addition, it will be possible to easily extend the traces section by
adding some new directives.