Replace flow reference counter with memory safe implementation

Flow’s reference counted types come into being as a raw pointer with a reference count of 1. Wrapping them in additional references does not increase the reference count. Instead, callers are expected to manage the references by invoking addRef manually. Failure to do so leads to use after free errors.

Modern C++ reference counted types maintain the invariant that, in the absence of pointer arithmetic or manual calls to delete, all live Reference objects point to a valid object. We should establish this invariant by modifying or replacing the Flow ReferenceCounted type.

Also, Flow includes a compile time option (which is disabled) to make its ReferenceCounted objects threadsafe. It also includes a ThreadSafeReferenceCounted type. The latter is used in a few places in the code base. It is generally difficult to write (or understand) thread safe code that relies on racing access and deallocation of objects. As part of cleaning this up, we should examine (and perhaps rewrite) those call sites.