Align ICache.TryRemove overloads with ConcurrentDictionary

[bitfaster]

Provide functional parity with ConcurrentDictionary for ICache.TryRemove.

• Provide bool TryRemove(K key, out V value) and bool TryRemove(KeyValuePair<K,V> item)
• Underneath, use ConcurrentDictionary TryRemove(key, out value) for ICache TryRemove(K key) and TryRemove(K key, out V value) to avoid double dictionary lookup then remove.
• Use the old method of retrieve, check, then atomic delete for TryRemove(KeyValuePair<K,V>)
• ICache 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 ICache themselves, upgrades the library, and doesn't have impls for these methods.
• Atomics implement IEquatable to compare wrapped values. Values that are not yet created are always not equal, similar to comparing null.
• AsyncAtomic family can follow a similar pattern, but this is not implemented in this PR.
• Defer support for scoped caches, less clear how to implement this.

[coveralls]

coverage: 97.182% (+0.009%) from 97.173% when pulling cc1e338 on users/alexpeck/tryrem into a112f23 on main.