Make node role verification optional via :verify_role option.

Not verifying gets rid of redundant INFO query before every redis operation.

Relatedly: Slaves are read-only by default since Redis 2.6, via 'slave-read-only yes' config option, so any invalid writes would be protected anyway.

Note: This is essentially the same as https://github.com/ryanlecompte/redis_failover/pull/44 , but makes verification a config option (defaults to enabled).