cleanup builders

[bitfaster]

Name builder types with a consistent order:

async => cache
scoped => async
atomic => scoped, async

Rename builder extension methods so that methods changing the cache type have prefix 'As', and configuration methods have prefix 'With', e.g.

var lru = new ConcurrentLruBuilder<int, Disposable>()
   .WithAtomicValueFactory()
   .AsAsyncCache()
   .AsScopedCache()
   .WithCapacity(3)
   .Build();

Add descriptive comments to the builder extension methods making it easier to use.