Mitigate write lock contention in reading CSV

[msk]

Reader::next_batch in reader.rs has the following:

let mut entry = map.entry(key.to_string()).or_insert_with(|| {
    self.enum_max_values.get(&i).map_or(
        (u32::max_value(), 0_usize),
        |v| {
            let mut value_locked = v.lock().expect("safe");
            if *value_locked < u32::max_value() {
                *value_locked += 1;
            }
            (*value_locked, 0_usize)
        },
    )
});

DashMap::entry tries to acquire a write lock on the underlying HashMap, waits until the lock becomes available. Since most of the time the routine above won't insert a new entry, attempting to get a write lock every time is a waste. Better to change it to a read lock for common cases.