SEGFAULT after `cluster.close()`

Scope, Collection and Bucket classes holds a reference to Connection. When all the connections are closed on the cluster, these classes mantains the reference to a destroyed connection, causing a SEGFAULT.

```javascript 

const cluster = new cb.Cluster('...');
const bucket = cluster.bucket('default');
const collection = bucket.defaultCollection()

const r1 = await collection.get('...')
console.log('response 1', r1)

await cluster.close(); // close all connections

//await cluster._connect() // <--- SEGFAULT here

const r2 = await collection.get('...') // <--- SEGFAULT here
console.log('response 2', r2)

```