remove .NET 4.0 from build system as we are no longer targetting it.
fixup some mono related issues and refactored how address family gets handled because linux does not support DualMode sockets.
updated nuget to support restoring via solution file and added the restore into the build scripts.
removed unused variable.
fixup issue with multiple versions of nuget packages.
CSHARP-875: Remove everything from MongoDB.Driver that has been moved to Core (temporarily replaced with a lot of NotImplementedExceptions). Use IClusterHandle to track lifetime of clusters referenced by MongoClient.