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
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