Explictly check return from lock functions

Currently the postgres adapter uses connection#select_value which looking at the documentation the call chain is :

select_value -> select_one -> select_all

select_all "... returns an array of hashes ..."
select_one "... selects first hash of array ..."
select_value " ... selects first value of hash ..."

This means that when selecting more than one value in a statement you need to explictly request which column you want.

In some Rubies maybe select_value always returns the output of the lock functions but at least in REE/MRI 1.8 and jRuby in 1.8 mode where I tested this is not true.

The fix is simple - use execute and retrieve the value ( PR coming ) or change the SQL to only select the lock function result.