rgw: collect skips a specific coroutine stack

[yehudasa]

Fixes: http://tracker.ceph.com/issues/16665

Instead of drain_all_but() that specifies number of stacks to leave behind,
added drain_all_but_stack() that has a specific stack specified. This is needed
so that we don't call wakeup() through lease_cr->go_down() on a cr stack that
was already collected.

Signed-off-by: Yehuda Sadeh yehuda@redhat.com

[cbodley]

looks good. only suggestion would be to make the skip_stack argument optional (default to nullpr)

[yehudasa]

@cbodley yeah, I wanted to make sure that I visited all call sites when going through the change

[cbodley]

I think we should be holding a reference to these lease_stacks. Even though we only use it to check for pointer equality in collect() and never dereference, it's possible for it to be freed and reallocated by another spawn(). This would cause us to skip the wrong stack