[6.x] Add LazyCollection@remember method

[JosephSilber]

This PR adds a remember method to the LazyCollection class.

Calling remember returns a new lazy collection that will remember any values that are enumerated, and will not pull it again from the source when it's enumerated again. Here are two examples:

Example 1

$users = User::cursor()->remember();

// No query has been executed yet.

$users->all();

// All values have been pulled from the DB.

$users->all();

// We did not hit the DB again. We got the users from `remember`'s cache.

Example 2

$users = User::cursor()->remember();

// No query has been executed yet.

$users->take(5)->all();

// The query has been executed, and the first 5 users have been streamed from the DB.

$users->take(20)->all();

// The first 5 users came from the cache. The rest continued the stream from the DB.

This PR includes extensive tests ensuring that it's fully lazy, and that two simultaneous runners don't step on each other's toes.

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The partition method

After this is merged, I'll rewrite the partition method using this.

Currently, calling partition immediately enumerates the whole collection, since we had no way to create two streams for the same dataset.

Now with remember, we can get a truly lazy version of partition!

[taylorotwell]

Just being honest these method names on LazyCollection seem so hard to figure out. When I see "remember" i think it has something to do with Laravel's caching system.

[danilopinotti]

Just being honest these method names on LazyCollection seem so hard to figure out. When I see "remember" i think it has something to do with Laravel's caching system.

What about 'keep' or 'retain'?

[tobiasthaden]

Obviously a breaking change but – what about make remember the default and add a fresh() method?

[Miguel-Serejo]

How about just ->lazy()?

[n10000k]

partition() ? I love this idea !

[Arkanius]

I like the name "remember" because I can figure out that this is using cache like the functions of cache system

[netpok]

@tobiasthaden I think that would lead to a lot of confusion, and in my case I would probably write fresh more than remember with the current implementation.

@36864 It's an already existing method on the collection so that's not a good solution in my opinion.

collect()->lazy()->lazy();

@narwy That's already an [existing method])https://laravel.com/docs/6.x/collections#method-partition)

I like the remember, I don't see the reason to use different names in different parts in the software, just like we don't use different methods for dispatching a job or an event.

I want cache to remember something: cache()->remember();
I want the collection to remember somethin: $collection->remember()

[taylorotwell]

I'll add it but I can't say that I understand it, heh. 😬

[driesvints]

@JosephSilber thanks! Can you also send in a pr to the docs?