Rename config.h to session-config.h

The global config.h is included on the command line with
the gcc '-include' switch which searches the local directory
before the include path. This leads to a namespace collision
with 'src/common/config/config.h'.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>