Allow yet more file handles for TokuDB.

Also revert to REPEATABLE-READ as TokuDB seems to have some issues with lesser
isolation levels such as a "race in secondary index query" bug... er, feature.

Technically the link below refers only to READ-UNCOMMITTED while the issue
occurred on dbstore1002 when using READ-COMMITTED. So, needs more research.

http://www.tokutek.com/2013/11/what-does-the-incorrect-key-file-for-table-error-mean/

Change-Id: Ide92510aeccedc1a6a699c51e681161a0d5006d3