If we have integer scores on the sorted set we're not using the fastest way
to reply by calling `d2string` which uses `double2ll` and `ll2string` when it can,
instead of `fpconv_dtoa`.
This results by some 50% performance improvement in certain cases of integer
scores for both RESP2 and RESP3, and no apparent impact on double scores.
Co-authored-by: Oran Agra <oran@redislabs.com>