FIX: Off-by-one error setting the distributed mutex key to expire

Accounting for fractional seconds, a distributed mutex can be held for
almost a full second longer than its validity.

For example: if we grab the lock at 10.5 seconds passed the epoch with a
validity of 5 seconds, the lock would be released at 16 seconds passed
the epoch. However, in this case assuming that all other processing
takes a negligible amount of time, the key would be expired at 15.5
seconds passed the epoch.

Using expireat, the key is now expired exactly when the lock is released.

lib/distributed_mutex.rb

@@ -95,7 +95,7 @@ class DistributedMutex
         result =
           redis.multi do
             redis.set key, expire_time.to_s
-            redis.expire key, validity
+            redis.expireat key, expire_time + 1
           end
 
         got_lock = !result.nil?