Migrate collection attributes to use timedelta for durations

The SDK2 Bucket attributes, such as Client.timeout, Client.views_timeout, Client.compression, tracing settings, etc are now couchbase_core.client.Client attributes.

Note that these settings are per LCB instance so currently that is per Collection, but after we start sharing instances it will be per Bucket. If one adds the converson from timedelta to 'float in seconds' at the couchbase_core.client.Client level then that should be relatively future-proof in this respect, although we will still need to rework the Bucket class to expose the properties on the relevant Bucket instance.

Also update migration guide/other docs.