Config option to bypass Redis server

Hey. This kind of goes off the idea introduced by ```withOptionalRedis { ... }``` in the ```RedisService```. If the Redis server is down it will currently spit out a full stacktrace to the error log and proceed as if it were a cache miss. If the Redis server is down for an extended amount of time the stacktraces will start to pile up. 

I've added a new, optional config property  ```grails.redis.bypassServer``` which when set to ```true``` will default to cache miss without throwing the stacktrace.

I added an integration test which shows the missed cache closure being called both times on ```redisService.memoizeDomain(...)``` by setting ```grails.redis.bypassServer=true```. I've also updated the documentation to include the new config property.