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>
config: note which options are overridden by common_preinit()

Defaults for these differ based on the context in which they're used.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge remote-tracking branch 'gh/wip-log-max' into next

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