perf session: Add facility to peek at all events

AUX area samples are not limited in how far back in time the sample
could start. Consequently samples must be queued in advance to allow for
time-ordered processing. To achieve that, add
perf_session__peek_events() that walks and peeks at all the events.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20191115124225.5247-11-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>