[change] Renamed celery monitoring tasks with leading underscores #569

[pandafy]

Fixes #569

Checks:

• I have manually tested the proposed changes
• I have written new test cases to avoid regressions (if necessary)
• I have updated the documentation (e.g. README.rst)

[nemesifier]

This is not what I meant.
I did not mean to change the name.

I meant to swap the names of _timeseries_write and timeseries_write.
I think the celery task should be named timeseries_write and the function used internally in the application should be named _timeseries_write, while now is the opposite.