trace2: write discard message to sentinel files

trace2: write discard message to sentinel files

Add a new "discard" event type for trace2 event destinations. When the
trace2 file count check creates a sentinel file, it will include the
normal trace2 output in the sentinel, along with this new discard
event.

Writing this message into the sentinel file is useful for tracking how
often the file count check triggers in practice.

Bump up the event format version since we've added a new event type.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>