state/multiwatcher: move Delta type to this package
cleanup
Merge pull request #1108 from davecheney/211-state-multiwatcher-delta

state/multiwatcher: move Delta type to this package

This is the first part of the post #1103 refactoring. 

This PR moves the Delta type, which was originally from `apiserver/params` to `state/multiwatcher`. The small size of this diff suggests that this is the correct place for this type.