(Original title: "find/cursor documentation doesn't warn about client lifetime")

From https://groups.google.com/forum/#!topic/mongodb-user/girvbR21AzM:

auto c = pool->acquire()->database()[collection].find({});
// loop c

pool->acquire() returns an unique_ptr, and cursor object c rely on this unique_ptr, before c destroy, unique_ptr need stay.
This is dangerous, however user may never find this bug until they release into production env.
Maybe add an assert to check this, let it crash early.