Motivation
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Each database served keeps a full registry in a process-wide LRU. The LRU is bounded by a count, so on a server hosting thousands of databases the number of retained registries follows traffic rather than memory pressure. Their combined footprint can push a worker past its virtual-memory soft limit, at which point it is killed and restarted.

On a server with ~2500 databases, the soft limit is reached at ~180 resident databases while the LRU could still hold ~210, so HTTP workers were being recycled under normal load.

Tracking usage
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Every request for a registry goes through the single lookup in the registry constructor, which stamps it with a monotonic timestamp; the stamp is also set when a registry is first built.

Collecting idle registries
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A collection pass drops every registry whose last use is older than the configured idle timeout. It runs at the end of registry loading, so it fires periodically as databases come and go.

Registries that are still loading are skipped, so a concurrent build is never collected. Dropping a registry only detaches it from the LRU: a request still holding a reference keeps working, and the next lookup rebuilds it.

The timeout is read from ODOO_REGISTRY_MAX_IDLE_TIMEOUT, in seconds; a value of zero, the default, disables the mechanism so behaviour is unchanged unless it is opted into.

Results
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With a five-minute timeout on the same ~2500-database server, the HTTP workers settle at around 40 resident registries instead of saturating memory on the long run. The gevent worker, which sees every web client reconnect at startup and briefly fills the LRU with ~150 databases, releases most of them on the first pass, reclaiming the memory.

On a real-life SaaS server with 64GB of RAM, that frees up to ~10GB which were previously taken by unused registries in the LRU. It comes at the expense of extra registry recomputes, but on the other hand workers do not reach their virtual memory limit anymore.

closes odoo/odoo#276581

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




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Forward-Port-Of: odoo/odoo#278236