Propose a fix for globalsign#120

As discussed in the issue globalsign#120, isMaster() can cause a
deadlock with the topology scanner if the connection it makes dies
before running the command; mgo automagically attempts to make another
socket in acquireSocket, but this can't work without topology.

This commit forces isMaster() to actually run on the intended socket.

Author: KJTsanaktsidis

cluster.go

@@ -181,7 +181,7 @@ func (cluster *mongoCluster) isMaster(socket *mongoSocket, result *isMasterResul
 		})
 	})
 
-	err := session.Run(cmd, result)
+	err := session.RunOnSocket(socket, cmd, result)
 	session.Close()
 	return err
 }

session.go

@@ -848,6 +848,15 @@ func (db *Database) Run(cmd interface{}, result interface{}) error {
 	return db.run(socket, cmd, result)
 }
 
+// RunOnSocket does the same as Run, but guarantees that your command will be run
+// on the provided socket instance; if it's unhealthy, you will receive the error
+// from it.
+func (db *Database) RunOnSocket(socket *mongoSocket, cmd interface{}, result interface{}) error {
+	socket.Acquire()
+	defer socket.Release()
+	return db.run(socket, cmd, result)
+}
+
 // Credential holds details to authenticate with a MongoDB server.
 type Credential struct {
 	// Username and Password hold the basic details for authentication.
@@ -2312,6 +2321,13 @@ func (s *Session) Run(cmd interface{}, result interface{}) error {
 	return s.DB("admin").Run(cmd, result)
 }
 
+// RunOnSocket does the same as Run, but guarantees that your command will be run
+// on the provided socket instance; if it's unhealthy, you will receive the error
+// from it.
+func (s *Session) RunOnSocket(socket *mongoSocket, cmd interface{}, result interface{}) error {
+	return s.DB("admin").RunOnSocket(socket, cmd, result)
+}
+
 // SelectServers restricts communication to servers configured with the
 // given tags. For example, the following statement restricts servers
 // used for reading operations to those with both tag "disk" set to