Introduce begin, rollback and commit

[fishcakez]

• Add begin(!)/1,2 to checkout and begin
• Add rollback(!)/1,2 to rollback and checkin
• Add commit(!)/1,2 to commit (or rollback) and checkin

rollback/2 is overloaded and has old behaviour inside transaction/3 for backwards compatibility. However transaction/3 is deprecated because is makes assumptions about database status that may not hold.

New return values from handle_begin/2, handle_rollback/2 and handle_commit/2 allow to signal mistaken database status.

[fishcakez]

This change is backwards compatible, so the plan is to soft deprecated transaction/3.