Add perf_evlist__parse_sample_timestamp to retrieve the timestamp of the
sample.

The idea is to use this function instead of the full sample parsing
before we queue the sample. At that time only the timestamp is needed
and we parse the sample once again later on delivery.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-o7syqo8lipj4or7renpu8e8y@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>