Implement Oracle SERIALIZABLE + real read of isolation level

Implement Oracle SERIALIZABLE + real read of isolation level

There's some significant awkwardness in that we can't
read the level unless a transaction is started, which normally
does not occur unless DML is emitted.  The implementation
uses the local_transaction_id function to start a transaction.
It is not known what the performance impact of this might
have, however by default the function is called only once
on first connect and later only if the get_isolation_level()
method is used.

Fixes: #5755
Change-Id: I0453a6b0a49420826707f660931002ba2338fbf0