advisory_lock_exists?

We needed to check if the advisory lock exists from within the block so we added this method.

```ruby
do_something unless Tag.advisory_lock_exists?("foo")
```

I only added it to the `PostgreSQL` subclass. I wasn't able to get the test suite running on this machine, so I didn't add a test for it but did verify it works right. I also added the method to `Base` which raises a `NoMethodError` if it's used in other DB implementation classes.