In some modules it may be useful to have an idea about being near to
OOM. Anyway additionally an explicit call to get the fill ratio will be
added in the future.

Plus freeMemoryIfNeeded() refactoring to improve legibility.
Please review this commit for sanity.

This way it is possible to use conditional compilation to be compatible
with a larger amount of Redis versions, however note that this breaks
binary compatibiltiy, so the module must be compiled with the
corresponding redismodule.h file depending on the version of Redis
targeted.

Note that this was an experimental API that can only be enabled with
REIDSMODULE_EXPERIMENTAL_API, so it is subject to change until its
promoted to stable API. Sorry for the breakage, it is trivial to
resolve btw. This change will not be back ported to Redis 4.0.

While this feature is not used by Redis, ae.c implements the ability for
a timer to call a finalizer callback when an timer event is deleted.
This feature was bugged since the start, and because it was never used
we never noticed a problem. However Anthony LaTorre was using the same
library in order to implement a different system: he found a bug that he
describes as follows, and which he fixed with the patch in this commit,
sent me by private email:

    --- Anthony email ---

've found one bug in the current implementation of the timed events.
It's possible to lose track of a timed event if an event is added in
the finalizerProc of another event.

For example, suppose you start off with three timed events 1, 2, and
3. Then the linked list looks like:

3 -> 2 -> 1

Then, you run processTimeEvents and events 2 and 3 finish, so now the
list looks like:

-1 -> -1 -> 2

Now, on the next iteration of processTimeEvents it starts by deleting
the first event, and suppose this finalizerProc creates a new event,
so that the list looks like this:

4 -> -1 -> 2

On the next iteration of the while loop, when it gets to the second
event, the variable prev is still set to NULL, so that the head of the
event loop after the next event will be set to 2, i.e. after deleting
the next event the event loop will look like:

2

and the event with id 4 will be lost.

I've attached an example program to illustrate the issue. If you run
it you will see that it prints:

```
foo id = 0
spam!
```

But if you uncomment line 29 and run it again it won't print "spam!".

    --- End of email ---

Test.c source code is as follows:

    #include "ae.h"
    #include <stdio.h>

    aeEventLoop *el;

    int foo(struct aeEventLoop *el, long long id, void *data)
    {
	printf("foo id = %lld\n", id);

	return AE_NOMORE;
    }

    int spam(struct aeEventLoop *el, long long id, void *data)
    {
	printf("spam!\n");

	return AE_NOMORE;
    }

    void bar(struct aeEventLoop *el, void *data)
    {
	aeCreateTimeEvent(el, 0, spam, NULL, NULL);
    }

    int main(int argc, char **argv)
    {
	el = aeCreateEventLoop(100);

	//aeCreateTimeEvent(el, 0, foo, NULL, NULL);
	aeCreateTimeEvent(el, 0, foo, NULL, bar);

	aeMain(el);

	return 0;
    }

Anthony fixed the problem by using a linked list for the list of timers, and
sent me back this patch after he tested the code in production for some time.
The code looks sane to me, so committing it to Redis.

There are too many advantages in doing this, RDB is faster to persist,
more compact, much faster to load back. The main issues here are that
the code is less tested because this was not the old default (so we are
enabling it for the new 5.0 release), and that the AOF is no longer a
trivially parsable format from now on. However the non-preamble mode
will be supported in the future as well, if new data types will be
added.

Update object.c

Some commands will udpate lfu-counter many times in one call.

Merge pull request #4722 from charsyam/feature/refactoring-call-aeDeleteFileEvent-twice-in-freeClusterLink

Refactoring to call aeDeleteFileEvent twice as once

anet: avoid double close

Make blocking list commands send keyspace notifications