Fix precision loss.

Large delta values often imply a difference between a large base value
and the large delta value, potentially resulting in small numbers with
a huge precision error. Since large delta values need 8 bytes anyway,
we are not even saving memory.

As a solution, always save the absoluto value rather than a delta once
8 bytes would be needed for the delta. Timestamps are then saved as 8
byte integers, while values are always saved as float64 in that case.

Change-Id: I01100d600515e16df58ce508b50982ffd762cc49
Remove float diff tolerance altogether.

Change-Id: I9ea9683a4665d5800fca75560bb4b8a8b4406d55
Merge pull request #401 from prometheus/exp/fix-precision-problems

Fix precision loss.