Add optional parameter to LPOP/RPOP to pop more than 1 element

[torwig]

Related TCL-test were copied from the Redis repository.
Method List::Pop refactored: output param std::string *elem became the last one for convenience purposes.

Please let me know if something should be checked additionally.

[git-hulk]

Thanks @torwig, we can use popMulti to implement the pop as well? since I think pop is the special condition for popMulti.

[torwig]

@git-hulk I had a similar idea, but then I read the Redis docs:

When called without the count argument:
Bulk string reply: the value of the first element, or nil ($-1) when key does not exist.
When called with the count argument:
Array reply: list of popped elements, or nil (*-1) when key does not exist.

and I saw that BLPOP/BRPOP and RPopLPush use just Pop without count option, so I decided to leave Pop for popping a single element and introduce PopMulti for popping more than one element.
Additionally, it's more explicit to return multiple elements from PopMulti inside a vector than to return just a single element inside a vector (if it's a special case for pop).

[git-hulk]

@torwig Oooop, my bad that didn't have a check at where's using pop. Current implementation is good to me.

[ShooterIT]

thanks @torwig Whatever i don't think it is a good idea to have PopMulti and Pop, we should implement Pop as a special PopMulti

[torwig]

@ShooterIT I replaced Pop implementation with multi-pop's one and pushed.
Or do you think I should implement Pop like this:

rocksdb::Status List::Pop(const Slice &user_key, bool left, std::string *elem) {
  std::vector<std::string> elems;
  auto s = PopMulti(user_key, left, 1, &elems);
  if (!s.ok()) return s;
  *elem = elems[0];
  return rocksdb::Status::OK();
}

?
And in this case, the code will be much cleaner.

[ShooterIT]

rocksdb::Status List::Pop(const Slice &user_key, bool left, std::string *elem) {
  std::vector<std::string> elems;
  auto s = PopMulti(user_key, left, 1, &elems);
  if (!s.ok()) return s;
  *elem = elems[0];
  return rocksdb::Status::OK();
}

I think this idea is better. WDYT @git-hulk

[torwig]

@git-hulk I've just pushed the version with the idea that I mentioned (Pop as PopMulti).
Here you can see the previous version with just a single PopMulti method in all the places: torwig@48ca0d9

[git-hulk]

cool, thanks @torwig

[git-hulk]

rocksdb::Status List::Pop(const Slice &user_key, bool left, std::string *elem) {
  std::vector<std::string> elems;
  auto s = PopMulti(user_key, left, 1, &elems);
  if (!s.ok()) return s;
  *elem = elems[0];
  return rocksdb::Status::OK();
}

I think this idea is better. WDYT @git-hulk

Yes, I think that retrieves multi elements in pop is a bit weird, it would be better to use popMulti to implement pop.

[git-hulk]

@ShooterIT Can you help to merge this PR?