Fix: sync event enablers before choosing header type

On session start, we should allocate the event IDs before figuring
out the number of events per channel and select the proper header
type.

Without this, the number of events is always perceived to be 0,
which selects the "compact" header type. For a channel containing
many events (e.g. enable-event -k -a), this selects an inefficient
header type. With this fix, it selects the "large" header type,
which is more appropriate for a larger number of event IDs.

This will lead to a reduced trace throughput for tracing workloads
that have many events.

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