Make ageOfLastShip, ageOfLastApplied extend TimeHistogram instead of plain histogram.

Currently age of last ship metric is an instance of an Histogram type. Here

ageOfLastShippedOpHist = rms.getMetricsRegistry().getHistogram(SOURCE_AGE_OF_LAST_SHIPPED_OP);

We can change it to TimeHistogram so that we get the range information also.