Optimization: don't use std::string in metadata #461
Description
metadata uses internally several instances of std::string to hold its string members. When performing benchmarks that query few rows with many fields, default-constructing these strings shows up in the profiler. Note that with metadata_mode::minimal (the default), no string copying takes place. Nonetheless, the cost is non-zero. We can change the strings to a std::vector and multiple offsets with no API change.