Adds: `L/RPOP <key> [count]`
Implements no. 2 of the following strategies:
1. Loop on listTypePop - this would result in multiple calls for memory freeing and allocating (see 769167a079)
2. Iterate the range to build the reply, then call quickListDelRange - this requires two iterations and **is the current choice**
3. Refactor quicklist to have a pop variant of quickListDelRange - probably optimal but more complex
Also:
* There's a historical check for NULL after calling listTypePop that was converted to an assert.
* This refactors common logic shared between LRANGE and the new form of LPOP/RPOP into addListRangeReply (adds test for b/w compat)
* Consequently, it may have made sense to have `LRANGE l -1 -2` and `LRANGE l 9 0` be legit and return a reverse reply. Due to historical reasons that would be, however, a breaking change.
* Added minimal comments to existing commands to adhere to the style, make core dev life easier and get commit karma, naturally.
f44186e575