osd: drop 'osd recovery max active' back to previous default (5)

osd: drop 'osd recovery max active' back to previous default (5)

Having this too large means that queues get too deep on the OSDs during
backfill and latency is very high.  In my tests, it also meant we generated
a lot of slow recovery messages just from the recovery ops themselves (no
client io).

Keeping this at the old default means we are no worse in this respect than
argonaut, which is a safe position to start from.

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