Stupid typo plus whitespace fixes.
It turns out I do need to override ->svp.

Since svp-at-top-level needs to start a txn, I need to make sure it does
so the way we want it to.

Since I've added an attribute (in_txn) which can't be supplied to ->new,
adjust the DDump filter.
Test the new ->txn and ->svp.

At least, test them as far as their interactions with DBC go. I need
proper tests for the retry logic later.

I also perhaps need to consider the possibility of failures a savepoint
can recover from. I don't believe there are any situations where rolling
Pg back to a savepoint and retrying exactly the same commands again will
help, but that might not be true of other databases. (Rolling back to a
savepoint will recover from a bad command, or from a constraint error,
but presumably those would just happen again.)
Avoid selectcol_arrayref.

For some reason I can't quite work out, it doesn't properly set the
sqlstate on the dbh. This means a thrown exception won't have the right
sqlstate, and the serial-retry logic won't work properly.