fix: move session support check to operation layer

Monitor checks run on a timer and network errors cause a reset
of the topology that would be corrected in the next cycle of the
monitor, we were checking a property that gets updated by this async
work. By moving the check to the operations layer we will allow users
to obtain a session regardless of support and then emit an error if
there is not support when the session is used in an operation.

NODE-3100

Author: nbbeeken

lib/mongo_client.js

@@ -457,10 +457,6 @@ MongoClient.prototype.startSession = function(options) {
     throw new MongoError('Must connect to a server before calling this method');
   }
 
-  if (!this.topology.hasSessionSupport()) {
-    throw new MongoError('Current topology does not support sessions');
-  }
-
   return this.topology.startSession(options, this.s.options);
 };
 

lib/operations/execute_operation.js

@@ -47,6 +47,8 @@ function executeOperation(topology, operation, callback) {
     } else if (operation.session.hasEnded) {
       throw new MongoError('Use of expired sessions is not permitted');
     }
+  } else if (operation.session && operation.session.explicit) {
+    throw new MongoError('Current topology does not support sessions');
   }
 
   let result;