Wrap modify-sql-fn to support returning a sequence

[snoe]

Still defaults to identity at call sites, but now will mapcat any commands that pass through it to be executed.

Allows turning "create table bar" into ["create table x.bar" "create table y.bar"] and executing both commands.

I tried changing proto/config to do the wrapping but it got way hairier than replacing the (or modify-sql-fn identity) sites with the wrap-modify-sql-fn call.

[yogthos]

Looks good, just pushed out 1.3.0 with the fix. 🎈