Call `Shutdown()` on all http servers. This means we will finish processing
any pending requests (including depsolving), but we will not listen to new
ones.
In particular, we will not answer to the readiness probe, so no new traffic
will be routed to this container.
Once all pending requests have been handled composer will shut down
gracefully and the liveness probe will return failure.
Note that in order for this to work correctly no requests should ever take longer
than the shutdown timeout (by default 30s).
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