NODE-1969

We should not default our change streams to a batch size of one,
but rather send no value and accept the server default.

NODE-2137

3.3.0 introduced an extra roundtrip of selection in order to
accurately determine if sessions were supported by the topology.
Unfortunately, the fix only supported callbacks. So, if a server
wasn't actually available immediately, we would return `undefined`
in cases when promises were requested for operation execution.

NODE-2136

The current deprecation warning tells users to pass this value to
`MongoClient.connect`, which is the static method on the client
class. This can be confusing to users, as they might assume that
the value should be passed to the `connect` method on the client
instance. Instead, we should guide users to pass the option to
the `MongoClient` constructor.

NODE-2138

Changed the sessions structure to an object instead of an array
so when we close a session we could remove it from the data structure more efficiently

Changed the sessions structure to a Set instead of an array
so when we close a session we could remove it from the data structure more efficiently