This started out as #2158 by sunheehnus, but I kept rewriting it
until I could understand things more easily and get a few more
correctness guarantees out of the readability flow.
The original commit created and returned a new ziplist with the contents of
both input ziplists, but I prefer to grow one of the input ziplists
and destroy the other one.
So, instead of malloc+copy as in #2158, the merge now reallocs one of
the existing ziplists and copies the other ziplist into the new space.
Also added merge test cases to ziplistTest()
This replaces individual ziplist vs. linkedlist representations
for Redis list operations.
Big thanks for all the reviews and feedback from everybody in
https://github.com/antirez/redis/pull/2143
