Improve testing.

In particular, create a fuzz test for time series.

Change-Id: I523a17912405a0b6b46bd395c781d201dfe55036
Improvements after review.

Change-Id: I484359282d4c7113518bbbb131f4f18383c08fdb
Merge pull request #400 from prometheus/exp/storage-ng-tests

Improve testing.