build async+atomic+scoped

[bitfaster]

Atomic builders do not support mixing sync and async (at least not in a way that would be safe if the caller mixes sync/async calls). Therefore, segregate the factory methods on ICache and IScopedCache so that sync and async calls cannot be mixed:

interface ICache { V GetOrAdd(K key, Func<K, V> valueFactory); }
interface IAsyncCache { Task<V> GetOrAddAsync(K key, Func<K, Task<V>> valueFactory); }
interface IScopedCache { Lifetime<V> ScopedGetOrAdd(K key, Func<K, Scoped<V>> valueFactory); }
interface IScopedAsyncCache { Task<Lifetime<V>> ScopedGetOrAddAsync(K key, Func<K, Task<Scoped<V>>> valueFactory); }

Extend LRU builder to support all combinations of:

• Metrics
• Expire after write (TLru)
• Scoped IDisposable values
• Synchronized factory method
• Async factory method

Use of builder now looks like this (methods can be called in any order with the same result):

var cache = new LruBuilder<int, Disposable>()
    .WithCapacity(666)
    .WithKeyComparer(EqualityComparer<int>.Default)
    .WithConcurrencyLevel(4)
    .WithAtomicCreate()
    .WithMetrics()
    .WithScopedValues()
    .AsAsyncCache()
    .Build();