Process queue items by time window

[aledbf]

This allows us to avoid building and compare a new Configuration (which is expensive) with the running configuration and cases where the sync is not required.

1. when inserting event to queue insert current timestamp with it (I will call it eventTimestamp)
2. just before taking any item from queue store current timestamp to a variable (I will call it takeoutTimestamp)
3. after taking item from queue complete all processing as usual. That will take some time but should leave the system synchronized at worst to the time of takeoutTimestamp
4. after synchronization is finished pop events from the queue as long as their eventTimestamp < takeoutTimestamp. That will get rid of all the events for changes that we are sure are already part of running state.

thanks to @Gregy for the logic ^^

fixes #1386

[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.1%) to 43.73% when pulling 768cbb8 on aledbf:group-queue-itema into 37262bd on kubernetes:master.