### Description
`MONGODB-AWS` authentication mechanism should follow SASL [spec](https://github.com/mongodb/specifications/blob/master/source/auth/auth.md#sasl-mechanisms). Current implementation ignores the `conversationId` sent by the server and uses a hardcoded integer for all `saslContinue` messages across all conversations.

#### What is changing?
Changing the hardcoded integer for `conversationId` to utilize the one that gets returned in the `saslStart` response.

##### Is there new documentation needed for these changes?
No. This change brings the implementation to spec.

#### What is the motivation for this change?

This is a bug. MONGODB-AWS authentication failed on database emulating MongoDB.

### Release Highlight

Fixes SASL implementation for MONGODB-AWS to use conversationId returned by the server instead of hardcoded integer.