added currentTimestamp() to store interface

[uweschaefer]

From @uweschaefer on 2019-06-20 13:33:20 +0000 UTC

see #457

Copied from original issue: Mercateo/factcast#458

[uweschaefer]

From @codecov[bot] on 2019-06-20 13:35:28 +0000 UTC

Codecov Report

Merging #458 into master will decrease coverage by 0.23%.
The diff coverage is 70.27%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
- Coverage   88.67%   88.43%   -0.24%     
==========================================
  Files          78       79       +1     
  Lines        1889     1911      +22     
  Branches      171      172       +1     
==========================================
+ Hits         1675     1690      +15     
- Misses        129      136       +7     
  Partials       85       85