Share lcb_INSTANCE objects between collections

This would mean we need to manually superimpose default timeouts on every Collection operation. This could theoretically be done by attaching the argument forwarder to each Collection with an associated timeout.

Sharing lcb_INSTANCEs between Collections would also mean we lose per-Collection control of other lcb_INSTANCE state such as tracing thresholds/timeouts, but at least the Response Time Observability RFC for SDK3 doesn't seem to explicitly mention a scope (though there are mentions of buckets and not collections), so this would seem to be compatible with the spec. There may be other issues with sharing lcb_INSTANCEs between collections but this is the obvious example.

If we decide to do this, we should update client-settings accordingly, to describe the scope of settings as 'couchbase.bucket.Bucket'. If not, we can refine the reference to 'couchbase_core.client.Client' to 'couchbase.collection.Collection'.