Change linkprops to always overwrite the existing value. (#912)

Given a schema:
```edgeql
type Target;
type SourceWithProp {
    multi targets: Target {
        lprop: int64;
    };
};
```

the following code was failing its assertion:
```py
target_a = default.Target()
self.client.save(target_a)

source = default.SourceWithProp(
    targets=[default.SourceWithProp.targets.link(target_a, lprop=1)]
)
self.client.save(source)

source.targets = [default.SourceWithProp.targets.link(target_a)]
self.client.sync(source)

assert all(
    target.__linkprops__.lprop == None
    for target in source.targets
)  # fail
```