Move Dataset#set to extension

I don't like the inconsistency between Dataset#set and Model#set.
Dataset#set is an alias for update and Model#set is quite different
than Model#update.
Deprecate Model.set, .update, .delete, and .destroy

These make it way to easy to require a database restore:

  album = Album[1]
  Album.destroy # instead of album.destroy

The thing is, this might not even be caught by tests, since the
album instance does get deleted from the database.
Remove Database#transaction :disconnect=>:retry

The :retry_on option is more general and should be used instead.
No default database for adapter/integration specs

The user should be required to provide one via an environment
variable or spec_config.rb.