Use the READ_COMMITED isolation level explicitly by default
Do not override a specified isolation level when creating a PgConnManager
Do not set isolation level when it is not necessary

This fixes some bad interaction between pgconnmanager and pgbouncer, which lead
queries to not be executed in the same connexion and so not in the same
transaction.
This problem appeared with newer versions of psycopg2 (2.4.2-1~bpo60+1)
release version 1.25.0

        * Use the READ_COMMITED isolation level explicitly by default
        * Do not override a specified isolation level when creating a
          PgConnManager
        * Do not set ISOLATION_LEVEL when it is not necessary