Revert "msgr: generate reset event on mark_down too"

This reverts commit accdfa67426129b298eb88fe8e664dfb6c855490.
msgr: generate reset event on mark_down to addr (not con)

If the caller is marking down an addr, they presumably don't have the
Connection* handy, so we should generate a reset event to help them
clean up con <-> session ref cycles.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: break con <-> session cycle when marking down old peers

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: update docs for mark_down, mark_down_all semantics

* RESET events
* note that the reset detection only happens if it is enabled in the
  policy.

Signed-off-by: Sage Weil <sage@inktank.com>