Ideally ::new() and ::setup() methods for devices would allow specification of feature and policy args

[mulkieran]

Instead, each type of device has a concept of a default table, which hard codes feature args. This should be changed, so that it is possible to use devicemapper to create, eg., two thinpools with different features.

[mulkieran]

Should probably just expose the appropriate Params object to the client, letting that be passed as an argument to setup() or new().