refactor(topology): always ensure useUnifiedTopology is present

This will be present if the `MongoClient` is created with this
option, but is not guaranteed to be there for direct uses of the
`Topology` type. The way the pool is currently set up, this could
result in `connect` not knowing the context of the topology its
creating connections for, so always make sure the variable is
present.

Author: mbroadst

lib/core/sdam/topology.js

@@ -33,6 +33,7 @@ let globalTopologyCounter = 0;
 
 // Constants
 const TOPOLOGY_DEFAULTS = {
+  useUnifiedTopology: true,
   localThresholdMS: 15,
   serverSelectionTimeoutMS: 30000,
   heartbeatFrequencyMS: 10000,