Remove references from disconnected models

[rubenswieringa]

Should occur when model relations are dissolved through changing of primary-key values.

• [ ] Disconnect when a model its foreign key is updated
• [ ] Disconnect when a model is destroyed

[rubenswieringa]

This issue addresses a situation where, for example:

hasOne 'crush', 'Person', key: 'crushID'
belongsTo 'admirer', 'Person', key: 'crushID', keyInSelf: false

Let's say romeo.crush == juliet (and consequently juliet.admirer == romeo) but then romeo.crush changes to caraDelevingne.

Currently juliet.admirer would still == romeo, but this issue should take care of that.