fix(server): Do not crash HRANDFIELD if some/all elements expired

[chakaz]

The issue is twofold:

1. If all elements are expired, dividing by Size() will crash. However, we cannot know this in advance, as begin() will actually invalidate all elements, but we're already inside RandomPair() which must return a result (it should not be called if real size is 0, after expiration)
2. We cannot use operator += before we know how many real elements are in the array. Today we do += rand() / Size(), and this in fact is problematic even if Size() is non-zero, but is greater than post-expiration size

Unfortunately, this solution will iterate over all elements in map for the multi-random retrieval function, to know how many non-expired elements (if any) are in the list. I don't think it's too bad, as we iterate there anyway (although not necessarily through all)

Fixes #2108 and other bugs.

[chakaz]

One such "more clever solution" would be to change the signature to optional<pair> RandomPair(), and also change the signature of Advance() to return whether it reached the end (or just check *this == end()), and then to change operator += to quietly exit if end() was reached.

However this is slightly subtle, and also I'm not a fan of quietly quitting without performing what we were asked for operator+=...

[chakaz]

I decided to, indeed, go with the slightly more complicated solution. It's not too bad, and definitely a better & safer API.
Will update the desc as well.

[romange]

maybe add a comment, this is O(n) function that may slow down RandomPairs for large sets?

[romange]

LGTM!

[chakaz]

friendly ping :)
(I only added a comment as you asked)