We need to allow our users to use redisContext->privdata as context
for any RESP3 PUSH messages, which means we can't use it for managing
SSL connections.
Bulletpoints:
* Create a secondary redisContext member for internal use only called
privctx and rename the redisContextFuncs->free_privdata accordingly.
* Adds a `free_privdata` function pointer so the user can tie allocated
memory to the lifetime of a redisContext (like they can already do
with redisAsyncContext)
* Enables SSL tests in .travis.yml
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
