This patch does the following:

1. Add a new method to acquire protection against global read lock,
   and call this (unless it is already done) before storing, updating
   or dropping dictionary objects, e.g. for resource groups, SRS, etc.

2. Add asserts to the DD API to make sure there is protection against
   the global read lock when storing, updating or dropping dictionary
   objects.

3. Implicitly acquire protection against the backup lock and
   global read lock when exclusive MDL is acquired from a SE.

4. Check for a valid cost_constant_cache before releasing a cost
   model entry. This is needed since the cost model is accessed
   from a background thread at a point where the cost model cache
   does not exist.

5. Introduce a DD api function to commit or rollback a transaction,
   and release transactional MDL.

Change-Id: I8fc7e1b38956f5816ba65cbdd66a996db314ac63