Add our own TransactionHook

The only addition so far is checking for an isolation_level decorator on
controller methods.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Update sample config to use our TransactionHook

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Support using a specified isolation_level

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Use isolation_level SERIALIZABLE for lock_many

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add test for IsolatedTransactionHook

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>