Fix: allow enabling same events for two channels

Fix: allow enabling same events for two channels

Fix the following use-case:

  593  lttng create
  594  lttng enable-channel test1 -u
  595  lttng enable-event -u -a
  596  lttng enable-event -u -a -c test1
  597  lttng start

run ./demo-trace

  598  lttng stop
  600  lttng view |wc -l

Previously, we were seeing only 8 events (number of events written in a
single stream), but now we see 16 events (each event is dispatched into
two streams).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>