Initialize MSSQL library when used

[michivi]

This PR changes pgloader's behavior so that it properly initialize freetds (through cl-mssql) when working with a MS SQL Server database.

This PR relies on the functions introduced in archimag/cl-mssql#11 (merged at the time of writing).

Before this PR, pgloader uses freetds directly without initialization. While the database copy does happen, users will see several instances of the following message:

Max connections reached, increase value of TDS_MAX_CONN

This happens because the freetds was not initialized properly. This PR initializes the library when a SQL Server is used.

Fixes #1354

[svantevonerichsen6906]

The debian build doesn't seem to have or pull the new cl-mssql (doesn't find the new init symbol).

[michivi]

This build issue comes from the fact that this change requires the latest version of cl-mssql, more specifically, this specific PR. That PR was only merged yesterday. So I guess, it's expected since the CI job downloads the official cl-mssql version from the Debian repository. So I guess that unfortunately, it means we'd have to wait for cl-mssql to be updated there?