Allows Sequential Compilation in DebugMode

## Tiny Problem

ServiceStack overwrites specified `CompileInParallelWithNoOfThreads` values.

Consider the following ServiceStack configuration code:
```cs
public class HelloAppHost : AppHostBase
{
    public override void Configure( Funq.Container container )
    {
        var razor = new RazorFormat
            {
                TemplateProvider =
                    {
                        CompileInParallelWithNoOfThreads = 0
                    }
            };
        
        this.Plugins.Add( razor );

        this.Routes.Add<Hello>( "/hello" );
        this.Routes.Add<Hello>( "/hello/{Name}" );
    }
}
```

I'd like ServiceStack to compile my Razor pages sequentially **while in DebugMode** to help simplify some debugging and page compilation errors.

Currently, **ServiceStack** ***overwrites*** my setting in the RazorPlugin's `Configure` method:

```cs
    public class RazorFormat : IRazorViewEngine, IPlugin, IRazorPlugin
    {
        public void Configure(IAppHost appHost)
        {
            this.AppHost = appHost;
            appHost.ViewEngines.Add(this);

            //Default to watching modfied pages in DebugMode
            if (!WatchForModifiedPages)
                WatchForModifiedPages = appHost.Config.DebugMode;

            //Default to parallel execution in DebugMode
            if (this.TemplateProvider.CompileInParallelWithNoOfThreads <= 0)
                this.TemplateProvider.CompileInParallelWithNoOfThreads = appHost.Config.DebugMode
                    ? Environment.ProcessorCount * 2
                    : 0;

```

In my case, sets `CompileInParallelWithNoOfThreads = 24` on my 6 core processor CPU with HT. :(

## Patch
This pull request resolves the issue by tweaking the `CompileInParallelWithNoOfThreads` into a nullable `int?` type so that values already set are not overwritten:

```cs
 if (!this.TemplateProvider.CompileInParallelWithNoOfThreads.HasValue)
                 this.TemplateProvider.CompileInParallelWithNoOfThreads = appHost.Config.DebugMode
                     ? Environment.ProcessorCount * 2
                     : 0;
```