JAVA-1395

This PR gets rid of the last type in org.mongodb: WriteResult.  The class has been moved to com.mongodb.WriteConcernResult.  The name change is for two reasons:

1. There is already a WriteResult class in com.mongodb
2. The .NET driver calls this class WriteConcernResult

The existing com.mongodb.WriteResult was moved out of driver-core and in to driver.  The only uses of it in driver-core were changed to use WriteConcernResult instead 

The only commit that was not a simple refactoring is the third one.