Log Message:
Update libpqxx to 7.1.2

Now requires c++17.

Headline:

   Easier, faster, neater: the transaction classes now have a stream
   method. You specify a query and the C++ types to which you want
   the fields converted, and you get to iterate over the rows. And
   thanks to C++14 structured bindings, you can read the fields
   straight into separate local variables:

   for (auto const [id, name]:
       tx.stream<int, std::string_view>("SELECT id, name FROM \ 
thing"))
   {
       process(id, name);
   }

   For super-fast access to a field's text-format contents, convert
   it to std::string_view. It'll give you an immediate reference
   to the buffer containing the field. Beware though: the buffer
   only holds that text for the one iteration. The next iteration
   will overwrite it.

Full list of changes at:
   https://github.com/jtv/libpqxx/blob/7.1.2/NEWS