[session/transaction] move most undo support into transaction A small part that need repo access has not been moved into transaction yet.

[session] simplify mode property We can delegate very thing to transaction utility

[session/transaction] pass the session object to the Transaction __init__ This simplify access to variables. As we are moving more and more logic and states in the Transaction it will greatly helps in future commit.

[session] use a dedicated class to track cnxset We introduce a new CnxSetTracker to track `cnxset` used by Transaction and allows to wait for them. This new class does not use Thread ID not thread joining to work. This allows to use multiple transaction per thread and a transaction in multiple thread. The class itself is totally threadsafe by the Transaction is still not thread safe. The old _threads_in_transaction attribute is dropped in favor of a new logic based on this object. The registration of cnxset used is not done by the Transaction itself. tx.cnset is a property handling the Consistency of its value with the CnxSetTracker instance. Note: The CnxSetTracker instance only track transaction id, not transaction itself, So not reference cycle are created.

[session] cleanup Transaction.cnxset The attribute should always be there. It is None when not cnxset exists.

[session] drop useless getter and setter for security Those function have very few user and does not anything fancy. Internal user now access tx.read/write_security directly. External user have been blessed with a writable property. They are only interested in the write part of they did not used the return of the removed function. They access it through the session since I prefer the external world to not be aware of Transaction yet.

[session/transaction] move read_security subtlety in transaction This is transaction data, itshould be handled by the transaction itself.

[transaction] initialize dbapi_request in __init__ There is not reason to have it implicitly declared. The attribute is renamed to match its public name.

[whatsnew] notify drop of deprecated code

[web/views] extract cube embed (closes #1916015)