Changed the version of the Histogram library to 2.0.0

[vetlewi]

In the PR there are several changes:

• Update the histogram library version (now v.2.0.0) - now the user doesn't need to run force_flush() manually for each histogram
• Change of queue type - the queue used to connect the steps in the pipeline has been changed to a more simple SPSC type. We also fix an issue where we would terminate the step in the pipeline prematurely (issue Not all data processed #27). We now check if the queue are empty before terminating the thread.