High Memory usage in gosnowflake

[rompetroll]

When streaming data to a snowflake stage using PUT, the driver allocates excessive amounts of memory. This is a known limitation snowflakedb/gosnowflake#536

To enable as many parallel uploads as possible, we should go back to using file writers instead of streaming. We should also gzip the files on disk though, so that we do not run out of disk as easily either.

[rompetroll]

Addressed in #10