Change ev_ to se_ in Tracker Protocol

[alexanderdean]

ev_ is legacy now that we have two different types of custom event. Therefore the five structured event fields (category, action etc) should start with se_ for structured event. So e.g. ev_ac -> se_ac.

Note that the Scalding ETL already supports both options, so once more people have moved onto the Scalding ETL, we can make the change.

This will require updating the JavaScript and Arduino trackers (not the No-JS Tracker).

[rgabo]

@alexanderdean, does this mean that you would standardise unstructured event fields with the usc_ prefix? IMHO ue_ might be a better choice as it is shorter and follows the same naming as se_.

Let's discuss and decide here and we can contribute the documentation change to https://github.com/snowplow/snowplow/wiki/snowplow-tracker-protocol#310-custom-unstructured-event-tracking.

[alexanderdean]

Agree - ue_ should be the prefix for unstructured events, short and to-the-point.

[alexanderdean]

Ah - just realised the Tracker Protocol says usc, hence your confusion! I have updated the Tracker Protocol to say ue now.

[alexanderdean]

Implemented as part of 0.8.7