Rename "event type" -> "event record type", "stream type" -> "data stream type"

Rename "event type" -> "event record type", "stream type" -> "data stream type"

This matches an eventual CTF 2 terminology and makes writing the barectf
documentation easier, as a data stream type describes data streams and
an event record type describes event records. An event record is what
the tracer records for an occurring event.

Throughout the code, for names, variables, and file names:

* `dst` means "data stream type".
* `ds` means "data stream".
* `ert` means "event record type".
* `er` means "event record".

The v3 YAML schemas are modified accordingly.

This patch also renames "discarded events counter" to "discarded event
records counter snapshot", as this field is a snapshot of the per-data
stream counter.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>