Add multiple keys support on EXISTS

[raghuiisc]

Redis EXISTS support multiple keys

As of now i see that hiredis-vip does not support the same. I get below error when i try the same.

ERROR: 4 Parse command error. Cmd type: 2, state: 10, break position: 24

From further debugging i see that in file "command.c" as following:

static int
redis_arg0(struct cmd *r)
{
switch (r->type) {
case CMD_REQ_REDIS_EXISTS: <<<<<<<

[bjosv]

HI!
We have solved this issue in our fork.
The PR that maybe could be reused is here:
Nordix/hiredis-cluster@746714d