Improve the performance of redisAppendCommandArgv

* Pulled in sdscatfmt from sds.c in Redis, and modified it to
  also take a size_t specific specifier.

* Pulled in sdsll2str and sdsull2str from sds.c in Redis (need by
  sdscatfmt).

* Changed redisFormatCommandArgv to use an sds string for command
  construction, rather than a char*.

Running a simple benchmark, I get the following results:

Old: 1.044806
New: 0.481620

The benchmark code itself can be found here:
https://gist.github.com/michael-grunder/c92ef31bb632b3d0ad81

Cheers,
Mike