Account for schema when checking whether a column exists

[brandur]

This one's aimed at addressing #503, in which migrations won't run when
using an alternate schema with search_path because the ColumnExists
query isn't accounting for schema in any way.

Here, modify ColumnExists so that it looks up within CURRENT_SCHEMA,
which will respect search path. Add test assertions to verify that this
works as expected, and similar ones in TableExists to avoid any
regressions there (it already does respect search path as written, but a
future change could potentially break that).

Fixes #503.

[bgentry]

Does this pollute the search path for this connection for future tests? Or does the conn pool get thrown away entirely after each test before the test DB goes back into the DB pool?

[brandur]

It gets rolled back with the transaction:

river_dev=# show search_path;
                 search_path
---------------------------------------------
 "$user", infra, logs, public
(1 row)

river_dev=# begin;
BEGIN

river_dev=*# set search_path = 'hello';
SET

river_dev=*# show search_path;
 search_path
-------------
 hello
(1 row)

river_dev=*# rollback;
ROLLBACK

river_dev=# show search_path;
                 search_path
---------------------------------------------
 "$user", infra, logs, public
(1 row)