OSD: add config option for peering_wq batch size

Large peering_wq batch sizes may excessively delay
peering messages resulting in unreasonably long
peering.  This may speed up peering.

Backport: cuttlefish
Related: #5084
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>