Facets histograms

[ktsaou]

This PR adds the ability for facets to generate histograms

• calculate histograms for all facet keys
• list the available histograms in the response
• add parameter histogram for the user to select the histogram he/she wants to render
• emulate a normal netdata chart using the histogram data

This is to be used for displaying a histogram above the systemd-journal function.

• The front-end can select any of the facets for histogram rendering.
• The chart is a normal netdata chart, supporting panning, zooming, etc.
• The chart is returned together with each function response, to be in sync with the data presented.