fix(util/rc): reference counter memory_order flags

See discussion at
http://www.chaoticmind.net/~hcb/projects/boost.atomic/doc/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters
http://stackoverflow.com/questions/10268737/c11-atomics-and-intrusive-shared-pointer-reference-count

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean): race condition

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>