[FIX] core: cursor hooks API and implementation

Python 3.8 changed the equality rules for bound methods to be based on
the *identity* of the receiver (`__self__`) rather than its *equality*.
This means that in 3.7, methods from different instances will compare
(and hash) equal, thereby landing in the same map "slot", but that isn't
the case in 3.8.

While it's usually not relevant, it's an issue for `GroupCalls` which is
indexed by a function: in 3.7, that being a method from recordsets
comparing equal will deduplicate them, but not anymore in 3.8, leading
to duplicated callbacks (exactly the thing GroupCalls aims to avoid).

Also, the API of `GroupCalls` turned out to be unusual and weird.  The
bug above is fixed by using a plain list for callbacks, thereby avoiding
comparisons between registered functions.  The API is now:

    callbacks.add(func)     # add func to callbacks
    callbacks.run()         # run all callbacks in addition order
    callbacks.clear()       # remove all callbacks

In order to handle aggregated data, the `callbacks` object provides a
dictionary `callbacks.data` that any callback function can freely use.
For the sake of consistency, the `callbacks.data` dict is automatically
cleared upon execution of callbacks.

Discovered by @william-andre

Related to odoo#56583

References:

* https://bugs.python.org/issue1617161
* python/cpython#7848
* https://docs.python.org/3/whatsnew/changelog.html#python-3-8-0-alpha-1
  (no direct link because individual entries are not linkable, look for
  bpo-1617161)

X-original-commit: a986f4929bab95759086cc77ca5e60795f82e91a

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