Allow ignoring SELECT from client

[rosmo]

Even if you are just using database 0, some developers want to call Redis' SELECT command always up front. This add a new redis_ignore_select option to pools to simply return OK for any SELECT command.

[Fgruntjes]

+1 would really like to see this merged, this prevents us from using twemproxy i.c.m. with Magento sessions.

[CLAassistant]

All committers have signed the CLA.