models/change: fix `id` property definition

Remove the flag `generated:true`, as it does not work together with
a custom `setter.id` function.
Merge pull request #633 from strongloop/fix-change-id-property

models/change: fix `id` property definition
2.4.1
Merge branch 'release/2.4.1' into production
Merge tag 'v2.4.1'

2.4.1
Register built-in models in a standalone file

Move the code registering built-in models to a new file
`lib/builtin-models.js`.
Dismantle `lib/models`.

 - Move core models `Model` and `PersistedModel` to `lib/`.
 - Move all other built-in models to `common/models`.

This is a preparation for extracting model definitions to JSON files.
By splitting the change into multiple commits, git is able to keep track
of file moves (renames).
Auto-load and register built-in `Checkpoint` model
test: remove infinite timeout

The infinite timeout was useful when debugging, which is not a good
reason for keeping it around when not debugging.
test: verify exported models
models: move User LDL def into `user.json`
models: move Email LDL def into `email.json`