Update --bigkeys to use SCAN

This commit changes the findBigKeys() function in redis-cli.c to use the new
SCAN command for iterating the keyspace, rather than RANDOMKEY.  Because we
can know when we're done using SCAN, it will exit after exhausting the keyspace.
Sentinel test: check role at end of unit 01.
Merge branch 'bigkeys_scan' of git://github.com/michael-grunder/redis into unstable