Return timestamp instead of datetime.datetime()

[maaktweluit]

ERROR [golemcli] wamp.error.invalid_payload: success return value from invoked procedure "comp.tasks" could not be serialized: WAMP message serialization error: can not serialize 'datetime.datetime' object

caused by #4920

[codecov]

Codecov Report

Merging #4938 into b0.22 will increase coverage by <.01%.
The diff coverage is 25%.

@@            Coverage Diff            @@
##           b0.22    #4938      +/-   ##
=========================================
+ Coverage   89.2%   89.21%   +<.01%     
=========================================
  Files        233      233              
  Lines      21462    21465       +3     
=========================================
+ Hits       19145    19149       +4     
+ Misses      2317     2316       -1