Prevent race condition where availableStreamIds isn't set

Ran into a race condition where `getInFlight()` was being called before `this.availableStreamIds` was set to an array, resulting in a JS error being thrown because `this.availabileStreamIds.length` wasn't set.

This was occurring in v1.0.0-beta1, but I based this fix branch off HEAD. Let me know if it'd be easier to base off the v1.0.0-beta1 tag.

Fuller stack:

```javascript
Uncaught TypeError: Cannot read property 'length' of undefined
      at Connection.getInFlight (XXXXX/node_modules/cassandra-driver/lib/connection.js:366:52)
      at XXXXXXXX/node_modules/cassandra-driver/lib/utils.js:178:24
      at Array.sort (native)
      at getLeastBusy (XXXXXXXXX/node_modules/cassandra-driver/lib/host.js:116:24)
      at fn (XXXXXXXXX/node_modules/async/lib/async.js:641:34)
      at Object._onImmediate (XXXXXXXXX/node_modules/async/lib/async.js:557:34)
      at processImmediate [as _immediateCallback] (timers.js:336:15)
```

Cheers.