Fix fc_atomic_ptr_get and use.

Before this change building with ThreadSanitizer and running
test/test-pthread generated a large number of threading issues. These
mostly stemmed from fc_atomic_ptr_get not doing an atomic load and using
"acquire load" instead of "load acquire". After making these changes it
was still necessary to use fc_atomic_ptr_get where it was needed.

This also documents the current memory barrier requirements for the
atomic primitives.
3 files changed