common: reduce default in-memory logs for non-daemons

The default of 100000 can result in hundreds of MBs of extra memory
used. This was most obvious when using librbd with caching enabled,
since there was a dout(0) accidentally left in the ObjectCacher.

refs: #4352
backport: bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 7c208d2f8e3f28f4055a4ae51eceae892dcef1dc)