Provide a way to change the DefaultMaxDegreeOfParallelism value programmatically

[mrward]

Can we provide a way to change the DefaultMaxDegreeOfParallelism used in various places in NuGet?

Having a large default value of 16 causes problems with restore in Xamarin Studio on Mono.

In the InstallCommand parallel processing is disabled on Mono:

https://github.com/NuGet/NuGet.Client/blob/250b20d83f57c687665f8682d80ef76efaaf5a00/src/NuGet.Clients/NuGet.CommandLine/Commands/InstallCommand.cs#L59

In other places this does not seem to be not possible since the DefaultMaxDegreeOfParallelism value is used directly.

To get around this problem in Mono I ended up changing the DefaultMaxDegreeOfParallelism constant to 1 and built NuGet from source. However it would be better if this could be modified or parallel processing disabled programmatically whilst this problem exists with Mono.

[rrelyea]

@zhili1208 - please investigate solution for this problem

[rrelyea]

@mrward - We weren't able to do this in 4.0. If still important, we'll consider in future.

[mrward]

Xamarin Studio has increased the number of thread pool threads available to it which seems to prevent the hang I was seeing before. However there does seem to be a problem restoring a lot of NuGet packages with Mono 4.8 and Xamarin Studio 6.2 which uses unpatched NuGet 3.5. Some of the socket connections seem to time out which I suspect is due to Xamarin Studio running out of thread pool threads during the restore. Running the restore again works presumably since only a few NuGet packages are being restored the second time.

Xamarin Studio 6.1 with a patched version of NuGet 3.4, which has DefaultMaxDegreeOfParallelism set to 1, and restoring a lot of NuGet packages works first time.