Hiredis: Advanced async-handling

[oshamash]

• hiredis.c: Convert redisSetError to take format and va_args
• dict.* : Fixed generic hash function -> cannot really take signed number
• async.* : Valdup function now handles OOM properly KeyCompare function uses proper sized integers nextArgument - change to use proper str-int conversion function Added redisAsyncAppend* function family to support buffer optimizations in async-mode Added proper error handling on OOM errors [inc buffer rollback]

Review fixes

• hiredis.c : Reverted back to memcpy(), will test *printf in different time
• aysnc.c : Minor changes for better readability

Handle unlikely/pathological OOM conditon cleanly.

More OOM checks

[oshamash]

@michael-grunder I took #822 (from #638) and decided to match to upstream, I think it's about time community will have access to ASYNC optimizations using the native hiredis client 😄

[michael-grunder]

I'm going to merge this but first I need to make sure that the change to dictGenHashFunction doesn't somehow cause pain in upstream Redis.

I think it's OK but we've had this kind of symbol conflict before (it's why all the sds symbols in hiredis are prefixed with hi_).