Align IAsyncCache.TryRemove overloads with ConcurrentDictionary

[bitfaster]

Provide functional parity with ConcurrentDictionary for IAsyncCache.TryRemove, following the implementation for ICache in #394.

• AsyncAtomicFactory implements IEquatable to compare wrapped values. Values that are not yet created are always not equal, similar to comparing null.
• Provide bool TryRemove(K key, out V value) and bool TryRemove(KeyValuePair<K,V> item) for IAsyncCache and AtomicFactoryAsyncCache.
• IAsyncCache default implementations for backwards compatibility throw NotSupportedException. There is no way to correctly implement either new method, so it is better to throw than give the caller an incorrect result. This would only apply if a caller implements IAsyncCache themselves, upgrades the library, and doesn't have impls for these methods.

[coveralls]

coverage: 98.678% (+0.07%) from 98.605%
when pulling 0cb52af on users/alexpeck/asynctryrem
into fe1a27a on main.