don't print qDebug information when it is a release build

if you want to print debug information when if is a release build
1) use qWarning instead
2) set env SEAFILE_CLIENT_DEBUG=true

debug build is not affected

what's more, separating log handlers for warning level brings some performance
improvement with the release build
Merge pull request #299 from haiwen/patch-29-dont-print-debug-when-release-build

don't print qDebug information when it is a release build