A similar fix was originally done in [1], where the access to `env` was done
before the parent dispatch.

The issue was then reintroduced with [2], where the `env` was possibly accessed
again after the dispatch. This works most of the time, but in the rare case
where the session is destroyed during dispatch, which is the case on
`/web/session/destroy`, accessing the environment after that point will crash.

This issue didn't manifest before [3], because the `env` was always initialized
during `checked_call` when calling the `clear` method on it (since `env` is a
magic property). After that commit, the `clear` is not called if not necessary,
therefore it might happen that the `env` is never initialized. This leads to the
crash when trying to initialize it for the first time after the `db` attribute
has been cleared during the destroy, since a `db` is required to initialize it.

The current fix aims to prevent the crash. As opposed to [1] that actually kept
the tracking fields by fetching them before the dispatch, it is decided on this
commit to voluntarily lose the tracking fields when destroying the session,
because keeping them would require too much refactoring for a fix in stable, but
we also feel that it makes sense functionally: those tracking fields were maybe
used for a specific purpose in the original database, but they might mean
something completely different on another database.

[1] 6780597f2d984a432341e70c8408de376c188fe0
[2] c78de22a098c810afa2c69a45220f83a8a8f45c0
[3] f6d56afba0ea96529b1419bf2ed4ac12fc5e98c1

closes #42260

Forward-Port-Of: odoo/odoo#42299

Before this commit nothing prevented high concurrency on a threaded http
server to consume too much resources, ending up failing requests either
because the OS is unable to spawn that many threads
(`RuntimeError: can't start new thread`), either because the Odoo db
connection pool is full (`PoolError: The Connection Pool Is Full`).

This commit adds the ODOO_MAX_HTTP_THREADS environment variable which
allows to limit the amount of concurrent socket connections accepted by
a threaded server, implicitly limiting the amount of concurrent threads
running for http requests handling.

Note that if a value has been provided to ODOO_MAX_HTTP_THREADS that cannot
be parsed as an integer, a value will be automatically set to half the
db connection pool size (which defaults to 64). This dynamic value is
chosen because while most requests will borrow only one cursor
concurrently, there are some exceptions where some controllers might
allocate two or more cursors.

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Forward-Port-Of: odoo/odoo#37238