RedisCache Remove Blocking IO on Event Loop Thread

[hmatt1]

The RedisCache constructor was making 2 calls that run the Redis command command to get a list of commands.

• new RedisAsyncCache();
• syncCommands(this.connection);

This blocking command is being run from a Netty Event Loop Group thread where the constructor is called, which causes problems since nioEventLoopGroup threads should not have any blocking calls run on them.

[hmatt1]

Just to post a little more background, the reason I started down this path is that we got a io.lettuce.core.RedisCommandTimeoutException when trying to instantiate the bean, and then I found this comment redis/lettuce#784 (comment) which sounded like the same thing.

Also, to update this PR with info from the conversation in the Gitter channel, @jameskleeh and I were discussing needing to make getCommands and getAsync thread safe, as well as if CompletableFuture.supplyAsync was the right thing to use to avoid blocking the event loop thread.

[graemerocher]

Thanks for the contribution