It's more natural to call the last entry added as "last", the original
commet got me confused until I actually read the code.

Most of the time to check the last element is the way to go, however
there are patterns where the contrary is the best choice. Zig-zag
scanning implemented in this commmit always checks the obvious element
first (the last added -- think at a loop where the last element
allocated gets freed again and again), and continues checking one
element in the head and one in the tail.

Thanks to @dvisrky that fixed the original implementation of the
function and proposed zig zag scanning.

Now that modules receive RedisModuleString objects on loading, they are
allowed to call the String API, so the context must be released
correctly.

Related to #3293.

Related to #3239.