Alkey

* fix major bug causing cache keys that should be different to match by adding
the `object_id` to the cache key argument, as well as the token value:
this means that keys generated with an instance argument will always be
unique to that instance (instead of previously matching an instance edited
in the same commit)

* bug fix model class write tokens
* provide (optional) `request.cache_manager`

* maintain table write tokens, e.g.: `alkey:users*`.

* namespace the global write token to `alkey:**`.

* only invalidate tokens in response to instance changes recorded within a
transaction that is committed successfully
* introduce the `'**'` global write token

Initial version.