osd: target transaction size 300 -> 30

Small transactions make pg removal nicer to the op queue.  It also slows
down PG deletion a bit, which may exacerbate the PG resurrection case
until #3884 is addressed.

At least on user reported this fixed an osd that kept failing due to
an internal heartbeat failure.

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