Optimize HRANDFIELD and ZRANDMEMBER case 4 when ziplist encoded (#8444)
It is inefficient to repeatedly pick a single random element from a
ziplist.
For CASE4, which is when the user requested a low number of unique
random picks from the collectoin, we used thta pattern.
Now we use a different algorithm that picks unique elements from a
ziplist, and guarentee no duplicate but doesn't provide random order
(which is only needed in the non-unique random picks case)
Unrelated changes:
* change ziplist count and indexes variables to unsigned
* solve compilation warnings about uninitialized vars in gcc 10.2
Co-authored-by:
xinluton <xinluton@qq.com>
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