[IMP] sql_db: default cursor = serialized, deprecate serialized_cursor, add doc

In the context of OpenERP transaction, we really need the
'snapshot isolation' level provided by PostgreSQL because
not being able to make repeatable reads within the same
transaction is a very dangerous source of corrupted data,
especially in a very concurrent system.
The performance impact of switching from the default
read_committed level to serializable/repeatable_read level
has been measured and and was not distinguishable from 
the standard deviation - so it seems quite acceptable.
User of PostgreSQL 9.1 might want to update to psycopg2
version 2.4.2 or later to properly use the regular 
snapshot isolation level, and not the newer, more
expensive serializable level (see comments in Cursor
docstring).

bzr revid: odo@openerp.com-20110714105552-9tgofrjtdgjmgc4b

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