Refactor IOperation.Write to IOperation.SendAsync

Returning a byte array from Write limits our options for memory management. We could refactor to an IMemoryOwner<byte>, but that puts the onus on the caller to handle disposing.

On the other hand, if we refactor "byte[] Write()" to "Task SendAsync(IConnection connection), this makes the interface simpler. Any kind of memory (byte array or ReadOnlyMemory<byte>) can be passed on to IConnection, without changing memory ownership. Additionally, the caller is no longer required to pass in op.Completed as a parameter to IConnection.SendAsync, the operation can provide this itself, which creates a cleaner API.