Cleanup conf module, reduce pylint errors
Add docstring for dbi module and its exceptions. Remove overriding of
RuntimeError constructor, because default behavior is to assign msg
parameter to self.message attribute.
Refactor dbi.Pool to use defaultdict and deque, because list.pop is
O(n) operation. Add docstring, reduce pylint errors.
Cleanup and document dbi.DB class
Cleanup and documnt dbi.dsn function
Cleanup code for execution of dbi as python script. Move it to a separate
function to keep global scope unchanged, make variable names more
readable and passing pylint checks.