Transaction isolation levels

[cbandy]

Most RDBMSs support the standard transaction isolation levels. Since we handle transactions in the API, it is probably worthwhile to support isolation levels there as well.

• SQLite does not seem to support isolation levels
• Should we raise an exception if requesting an isolation level while nested?
• Should we not raise an exception if the requested isolation level is less strict than the level of the current transaction?

http://www.postgresql.org/docs/current/static/transaction-iso.html
http://www.postgresql.org/docs/current/static/sql-set-transaction.html
http://dev.mysql.com/doc/en/commit.html
http://dev.mysql.com/doc/en/set-transaction.html