Add a load client

Command line interface more or less like dump client.

This does not offer any basic functionality above what you can do with PSQL, using COPY for CSV and MYSQL_DUMP files or \i for FDB SQL files.
It does do a little guessing of file formats and target tables.

More significantly, the load can be done in parallel using multiple threads. This requires breaking the file up into multiple pieces along line / statement boundaries, which is the only complicated code here. And, because of the need for that, everything uses nio.