Expiration time is not set atomically with the value #225
Description
When setting a value that has an expiration time, I noticed that redis-objects will first set the value, and in a separate operation set the expiration time. This is unfortunate not just for performance, but mainly because:
when setting the expiration fails (eg. due to network issues, ruby process interrupted, you name it), the value will never expire.
I have observed this problem in a production environment, where values would stick around forever. In our case, this was very problematic. Cache invalidation is a hard problem made easier by Redis (since it can be done atomically with the operation that sets the value). Unfortunately, redis-objects doesn't seem to make use of that.