Explicitly precompute GlobalId for reference values to avoid lazy init race

This is a workaround for `DocumentId::getGlobalId()`'s lazy GID computation
currently not being thread safe in a `const` context.

The following race was previously possible:
 * Attribute writer thread calls `DocumentId::getGlobalId` when applying
   the reference value update to a reference attribute. Writes GID and
   flag.
 * Shared executor thread calls `DocumentId::operator=` with the same ID
   instance as the rhs argument when applying the reference value update
   to the doc store. Causes a read race with the above write.

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