Make ALTER SERVER SET global like doc says.
Add a test.
Add a mode where the transaction manager is given a chance to commit from time to time.
Does nothing in the Persistit implementation.
merge mmcm: Make ALTER SERVER SET actually set property for all sessions.

https://code.launchpad.net/~mmcm/akiban-server/pg-server-set-variable/+merge/179828
merge mmcm: Add a mode where explicit transaction commits from time to time.

https://code.launchpad.net/~mmcm/akiban-server/pg-periodically-commit/+merge/179829