Delta histogram drop old aggregates

[tsloughter]

With a delta unrecorded metrics during a time interval shouldn't exported.

Example of what we get in a newly added failing test is recording to a histogram in the first time interval with 2 sets of attributes, then in the next interval not recording to the attributes #{status => 200,verb => <<"GET">>}, but we still get:

  expected: []
       got: [{#{status => 500,verb => <<"GET">>},[2],2,5,7}]
      line: 1378
   comment: [{#{status => 200,verb => <<"GET">>},
              [0],
              infinity,-9.223372036854776e18,0},
             {#{status => 500,verb => <<"GET">>},[1],5,5,5}]

[tsloughter]

Fixed by #677