Provide a cleaner API for working with pool entries

Currently, you need to use the * or -> operators on a pool::entry to use the underlying client. So it is a little awkward to use when doing client operations (e.g. obtaining a database), e.g. here:

auto coll = (*client)["test"][collection_names[static_cast<std::size_t>(j)]];

Perhaps instead, the pool::entry could also have an [] operator overload for obtaining the database?