public void setDestination(final Consumer<Path> destination) {
        this.destination = destination;
    }
Synchronised to stop closeOldAggregates being called concurrently (which
shouldn't really happen with the frequency executor), but
closeAggregate() will still happen under the striped lock to protect it
from other threads working on aggregates
stroom-proxy/stroom-proxy-app/src/main/java/stroom/proxy/app/handler/PreAggregator.java