experimental support for Pg loader run inside txn

experimental support for Pg loader run inside txn

To get this to work, there must not be any database errors thrown. So we
cannot use our usual technique of selecting from nonexistant tables and
throwing away the errors.

To that end, _table_comment and _column_comment in ::Loader::DBI now
check for the tables in $self->_tables before doing a query on them.

jnap added a test to the common tests in a previous commit which I
augment here to be $TODO for DBs other than Pg and to throw away the
usual rescan warnings.

There is also a warning for 'commit ineffective with AutoCommit enabled'
which we also throw away and is more worrisome, and will need further
attention, and is one of the reason this support is marked experimental.
That and this test failing for SQLite as well as some other DBs.