lib: introduce a read-write lock for northbound configurations

lib: introduce a read-write lock for northbound configurations

The upcoming gRPC-based northbound plugin will run on a separate
pthread, and it will need to have access to the running configuration
global variable.  Introduce a rw-lock to control concurrent access
to the running configuration. Add the lock inside the "nb_config"
structure so that it can be used to protect candidate configurations
as well (this might be necessary depending on the threading scheme
of future northbound plugins).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>