Improve message on table size retrieval error

There should be returned an actual self-describing message when errors
occur. This is one of many points that need errors being thrown and
appropriate messages returned.
Improve debug messages

The debug messages are now separated in a few levels and are prefixed
with a odbc_fdw:: tag that separates them from debug messages of other
parts os PostgreSQL.
Don't use database name when not informed

The database name is not mandatory, so it cannot be used unless
explicitly informed.
Correct database and schema optionality

The database and schema are optional configurations. If the database is
not provided, it should not be used on the connection string. If the
schema is provided, it should be used on the query instead of the
database.
Correct the closing of the connections

Either the connection has to be closed or it must be managed in order to
avoid exausting the connection limits of the server. The management was
not being done, so they must be closed.
On NULL, return NULL instead of "NULL"

On the array of strings to be turned into a tuple, the NULL value must
be represented as a NULL c-string, instead of a pointer to a string
containing the text "NULL".

based on:
commit 49c4564 (tureba/master)
Author: Arthur Nascimento <tureba@gmail.com>
Date:   Sun Nov 20 15:56:13 2011 -0200

Conflicts:
	odbc_fdw.c