Used symbolic constants for psycopg2 isolation levels.

Django used the value 1 = ISOLATION_LEVEL_READ_UNCOMMITTED in some places, but
PostgreSQL doesn't provide "read uncommited", it uses "read committed" instead:
http://www.postgresql.org/docs/9.1/static/transaction-iso.html. For clarity,
this commit uses ISOLATION_LEVEL_READ_COMMITTED = 2 where 1 was previously used.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@17112 bcc190cf-cafb-0310-a4f2-bffc1f526a37