core: move spinlock directly in the critical section function

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
core: simplify fast_ring interface

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
core: make core_memory_fence() not 'static inline' for profiling

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>