jtv/libpqxx 7.0.4

libpxx 7.0.4: More fixes, more constructors, more operators.

on GitHub

Another small batch of changes:

• Fix possible crash in connection::connection_string (#290).
• Fix filtering of default values in connection::connection_string (#288).
• Deprecate row::swap and public inheritance of iterators from row.
• More copy/move/default constructors/assignments on result iterators.
• More copy/move/default constructors/assignments on row iterators.