Add lock to scoped burdens to ensure only one instance is created per scope

[generik0]

Add lock to scoped burdens to ensure only one instance is created per scope

#533

[generik0]

@jonorossi can you add netfore3 and 3.1 to the appveyor? Then the tests can run in all frameworks

[generik0]

@AntonioDrusin can you review. it was the scopeCache that needed the lock. Hence we only sync for each scope. less performance hit, and more SOLID as scopeCache is an instance, not a static..

lock (scopeCache)
			{
				var burden = scopeCache[model];
				if (burden == null)
				{
					scopeCache[model] = burden = createInstance((_) => {});
				}
				return burden;	
			}

[AntonioDrusin]

The lock code looks correct. I tested with @mdavis test project and with my own. The resolvers now behave as expected.

[generik0]

@jonorossi improved nestling and improved naming of variables for readability

[generik0]

@jonorossi did i get all the review comments ok? If so, can we get this into master and a new build for people?

[jonorossi]

@generik0 yep, all good. Merged to master.