Redis New license wording

Hello, it was brought up in a previous thread what do we want the license information to look like (and do we need to update year/etc). I talked with some OSS folks within Amazon, and they suggested the following for the project:

/*
 * Copyright Redis Contributors. 
 * Copyright (c) 2018, Salvatore Sanfilippo <antirez at gmail dot com> (and everyone else that was already added explicitly)
 * All rights reserved.
 * SPDX-License-Identifier: BSD 3-Clause
 */

Let's quickly break this down: 1. Copyright Redis Contributors. was the recommended approach since Redis is not a legal entity ( Based on my understanding). See github history for details was also suggested but it isn't needed, it's just a clarifying statement. 2. It's apparently frowned upon to remove past contributors explicit attributions. For new files this would just be skipped and we would just include line 1. 3. All rights reserved. It was recommended we keep this 4. SPDX-License-Identifier: BSD 3-Clause. Was the recommended approach for replacing the content of the license. This is a semi-industry standard way for saying "This file is under this license". This is interchangeable with the full license text.

My recommendation is that new files use this header except the SPDX stuff. We should probably add Copyright Redis Contributors. to the COPYING file with See GitHub history for details. to clarify how to find the contributors. There wasn't really a strong inclination one way or another if we should go update all of the files. It won't mess up the blame, and might reduce some other confusion, so I would suggest we go add Copyright Redis Contributors. to all the files. The last bit, the SPDX, I don't see a strong reason to include it it.