config_opts.h: adjust recovery defaults

config_opts.h: adjust recovery defaults

osd max backfills: 5 was too low for a default, 10
 seems to work better in testing.  The message
 priority system should minimize disruption of
 push and pull operations anyway.

osd recovery max chunk: 1MB was too small for a
 default.  8MB is reasonable for a single push
 and will allow us to recover an rbd block in
 one push rather then 4 reducing client io
 latency during log-based recovery.

osd recovery op priority: 10 rather than 30 will
 further reduce the client io latency impact of
 push and pull operations.

Signed-off-by: Samuel Just <sam.just@inktank.com>