Add transaction possibilities

When executing several inserts and updates, using transactions is a must.

A macro within-transaction shall guarantee that any statement executed in its body is:

• Committed as one or rolled back in case of errors;
• Executed with the same connection

See cl-dbi:with-transaction for starters: https://github.com/fukamachi/cl-dbi/blob/eb3627895532ffe9291a3d51b2e9e09d34b7382b/src/dbi.lisp#L184