make sql_exporter buildable again, update go-mssqldb to latest version (go modules based)

[gebi]

Hi,

thx for your awesome exporter, it's a real timesaver for us. As we thought we had a connection problem we wanted to try with the latest go-mssqldb)

1. i've switched sql_exporter to use go modules
2. switched from promeotheus/golang-builder to golang:1.17 becuase the golang-builder image is 3.8GB in size
3. updated go-mssqldb to latest version

metrics are delivered just fine with the new version

(with the current master make docker does not build because of missing dependencies)