CSHARP-1043: Moved ReadPreference into Core.

Conflicts:
	src/MongoDB.Driver.Tests/MockOperationExecutor.cs
	src/MongoDB.Driver/IOperationExecutor.cs
	src/MongoDB.Driver/MongoClient.cs
	src/MongoDB.Driver/MongoCollectionImpl.cs
	src/MongoDB.Driver/MongoDatabaseImpl.cs
	src/MongoDB.Driver/OperationExecutor.cs