Fix multi-tenancy support such that it automatically propagates the tenantId for lazy loading

[rbygrave]

When lazy loading occurs we don't want to rely on the CurrentTenantProvider (which will often be ThreadLocal based) and instead have the tenantId provided by the "load context".

That is, when lazy loading occurs in "other threads" we don't want to have to rely on the CurrentTenantProvider and instead get the tenantId provided by the original "load context".