Avoid repeated reallocs growing the listpack while entries are being added.
This is done by pre-allocating the listpack to near maximum size, and using
malloc_size to check if it needs realloc or not.
When the listpack reaches the maximum number of entries, we shrink it to fit it's used size.
Co-authored-by: Viktor Söderqvist <viktor@zuiderkwast.se>
Co-authored-by: Oran Agra <oran@redislabs.com>
