rgw: include marker and truncated flag in data log list api

rgw: include marker and truncated flag in data log list api

Consumers of this api need to know their position in the log. It's
readily available when fetching the log, so return it.  Without the
marker in this call, a client could not easily or efficiently figure
out its position in the log, since it would require getting the global
last marker in the log, and then reading all the log entries.

This would be slow for large logs, and would be subject to races that
would cause potentially very expensive duplicate work.

Returning this atomically while fetching the log entries simplifies
all of this.

Fixes: #6615
Backport: dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit c275912509255f8bb4c854e181318b45ab0f8564)