WL#3430: Storage engine dedicated databases

Several bugs are present in the server when mixing
local tables and distributed tables in the same database.
These bugs are not possible to solve without storage engine
dedicated databases (i.e. to be able to specify distributed
databases by binding them to NdbCluster or some other distributed
storage engine).
However, in a migration scenario the possibility of mixing
is desirable, and also some mixing is done for performance
reasons in a system table used by cluster replication.
To support the possibility of enforcing totally distributed
databases, these have to be possible to tag with a storage engine:
CREATE DATABASE  STORAGE_ENGINE= [ONLY];
Setting the storage engine for a database should define the
default engine for a database (if none is specified in CREATE TABLE).
Trying to create a table in the database that is not belonging 
to the ONLY defined storage engine should consequently fail. 
Setting the current database as this database, i.e. with USE ;,
should overide the default storage engine setting.