create_database(), create_database_schema(): New functions

This allows us to avoid setting the search path to a non-existing schema.
database: Introduce factory for database creation
database: Add separate database connection for interning purposes

This is not used yet.
database: Run file attribute interning on the secondary database handle
database::add_dwarf(): Run DWARF producer interning on second handle
database: RPM metdata adding functions work on vectors now
database::add_package_dependencies(): Switch to COPY
database::add_package_scripts(): Use prepared statement
database::add_package_triggers(): Switch to COPY
database::update_contents_preview(): Switch to prepared statement
cxxll::rpm_file_info: Update outdated comment
cxxll::const_stringref::c_str(): Remove, it is too dangerous
cxxll::const_stringref::vec(): New member function
symboldb integration test: Check multiple ways to load the database