After a few attempts it looked quite saner to just add the last item ID
at the end of the serialized listpacks, instead of scanning the last
listpack loaded from head to tail just to fetch it. It's a disk space VS
CPU-and-simplicity tradeoff basically.

Related to #4483. As suggested by @soloestoy, we can retrieve the SHA1
from the body. Given that in the new implementation using AUX fields we
ended copying around a lot to create new objects and strings, extremize
such concept and trade CPU for space inside the RDB file.

See #4483. This is needed because luaCreateFunction() is now called
from RDB loading code outside a client context.

This is currently needed in order to fix #4483, but this can be
useful in other contexts, so maybe later we may want to remove the
conditionals and always save/load scripts.

Note that we are using the "lua" AUX field here, in order to guarantee
backward compatibility of the RDB file. The unknown AUX fields must be
discarded by past versions of Redis.

Doing the following ended with a broken server.executable:

1. Start Redis with src/redis-server
2. Send CONFIG SET DIR /tmp/
3. Send DEBUG RESTART

At this point we called execve with an argv[0] that is no longer related
to the new path. So after the restart the absolute path of the
executable is recomputed in the wrong way. With this fix we pass the
absolute path already computed as argv[0].

fix a typo

Fix some typos

redis-cli.c typo: helpe -> helper.

redis-cli.c typo: Requets -> Requests.

Fix typo

Fix typo

Update link to https and use inline link

See #4192, the original PR removed lines of code that are actually
needed, so thanks to @chunqiulfq for reporting the problem, but merging
solution from @jeesyn after checking, together with @artix75, that the
logic covers all the cases.

Correct spelling of "faield".

Fix file descriptor leak and error handling

Fix undefined behavior constant defined.

Splitting the popularity in half actually just needs decrementing the
counter because the counter is logarithmic.

Firstly, use access time to replace the decreas time of LFU.
For function LFUDecrAndReturn,
it should only try to get decremented counter,
not update LFU fields, we will update it in an explicit way.
And we will times halve the counter according to the times of
elapsed time than server.lfu_decay_time.
Everytime a key is accessed, we should update the LFU
including update access time, and increment the counter after
call function LFUDecrAndReturn.
If a key is overwritten, the LFU should be also updated.
Then we can use `OBJECT freq` command to get a key's frequence,
and LFUDecrAndReturn should be called in `OBJECT freq` command
in case of the key has not been accessed for a long time,
because we update the access time only when the key is read or
overwritten.