Fixes for Ubuntu bionic

Two fixes:

1. The behaviour of the standard C++ library has been fixed so that std::vector<>::clear() does not deallocate memory. It seems that previous versions of the library did deallocate, which is not standards conforming, but meant that a bug in planet_dump_ng went unnoticed and its memory usage exploded under the new version of the library.
2. The pg_restore tool from PostgreSQL now requires either a database name or file output argument. Previously the default was to output to stdout, but now this is a hard requirement.