Don't send SELECT + ROLE commands to Sentinels

[piotrp]

In #34 I added error handling in connect to properly handle errors after issuing SELECT, with a workaround for Sentinels (link). This PR provides a better solution for Sentinels, so that problematic SELECT call and subsequent ROLE aren't issued.

I did this in a way that allows API to be fully compatible, i.e. old workaround is still there, but internally we avoid issuing invalid command to Sentinel unless user tries to use connect("redis://sentinel-address"). To remove this we would need to alter public APIs in some way:

• default to db = ngx.null, and use SELECT only when db is provided - which would make resetting DB to 0 not happen when TCP connection is reused, unless requested explicitly by user
• add a new parameter to connection methods to handle connection initialization

[piotrp]

Hello @pintsized, just checking whether you remember about this ;)

[esatterwhite]

@pintsized Any word on this. would like see this get landed

[esatterwhite]

@pintsized Could we get this one published to luarocks?

[esatterwhite]

@pintsized nudge on this one? possible to publish this to luarocks?