[doc] Add 3.17 release notes

[migration] mark all version prior 3.7 as non-migrable API possible used by such old migration script are not supported in recent Cubiweb. Closes #2772958

[dataimport] drop checkpoint backward compat API Deprecated since 3.7. See #2772958 for details.

[dataimport] drop index support deprecated since 3.7 Dropping of pre-3.7 API decided in #2772958

[utils] drop pre-3.7 compat in util Dropping of API deprecated since 3.7 decided in #2772958

[migration] drop pre-3.7 utility Dropping of API deprecated since 3.7 decided in #2772958

[entity] drop pre-3.7 api on session Dropping of API deprecated since 3.7 decided in #2772958

drop dubious comment

[req] drop pre-3.7 api on request Dropping of API deprecated since 3.7 decided in #2772958

[session] drop pre-3.7 api on session Dropping of API deprecated since 3.7 decided in #2772958

[transaction] use set operation in the hook control code We use direct operator on set instead of looping by hand, This simplify the codes.

[session/transaction] move hook management in transaction itself This is transaction level code.

[session] allow writable tx_attr and use it for commit_state This keep clarifying the code.

[session/transaction] move add operation in session code

[session/transaction] move entity life utility on transaction This access transaction related data and therefor should live in transaction.

[transaction] small simplification in ecache code A single assignation is simpler.

[session/transaction] move entity cache management on session A new utility method is added for that. It is similar to the one used for attribute access. code is copy pasted as his. Function documentation is added.

[session] have a nice helper function to forward access to session This allows a lighter and clearer code. This will also ease the addition of deprecation warning in the future.

[session] drop dead code session._tx is never None. New transaction are created if necessary.

[session] use tx.data directly in session code.

[transaction] rename transaction_data to data Transaction.transaction_data is redundant. A transaction_data property is left for backward compatibility. Deprecation mechanism will come in later changeset

[session] document Session._tx

[session] split session creation from default session assignation This is the first step for more independence for Transaction.

[session] make session lock reentrant We are going to use it for other session related business. It is renamed from _closed_lock to _lock in the same move.