feat: perform DDL not in a transaction where possible

[NickCrews]

This allows the user to be in control of the transaction.

Now I can do

con.raw_sql("begin transaction")
con.insert("a", a)
con.insert("b", b)
con.raw_sql("commit")

and the two operations will be atomic. This isn't complete. Ideally we were consistent with this everywhere, but this is a good start for the operations I am doing.

[NickCrews]

Before I go through the effort of fixing the tests, I'd love a quick confirmation that this is going in the right direction.

[cpcloud]

Can't you use the Backend.begin() method for this? Or does that not work?