Proper support for RESP3 PUSH messages.
By default, PUSH messages are now intercepted and the reply memory freed.
This means existing code should work unchanged when connecting to Redis
>= 6.0.0 even if `CLIENT TRACKING` were then enabled.
Additionally, we define two callbacks users can configure if they wish to handle
these messages in a custom way:
void redisPushFn(void *privdata, void *reply);
void redisAsyncPushFn(redisAsyncContext *ac, void *reply);
See #825
