Adding the vertical split clone job. init() only works now.
Checkpoint for WIP on worker-based clone.
Moving the connection pool functionnality out of tabletserver.

Now used by tabletserver yet, need review first.
go fmt these files.
Now using the new dbconnpool in tabletserver.
Merge commit '047ffc2c84af0367207a003f1e99c285bce38650'