rgw/OutputDataSocket: actually discard data on full buffer

rgw/OutputDataSocket: actually discard data on full buffer

A dout message in OutputDataSocket::append_output() states that
data will be dropped when appending would cause data_max_backlog
to be exceeded--but the method appends it anyway.

Log output discards at level 0, as messages will be lost.  Suppress
repeated warnings mod 100.  Switch to vector.

Fixes: http://tracker.ceph.com/issues/40178
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit c806b825dae649829de8847d36cb21ffd2bbee8e)