Rewrite reference leak checker without finalizers.

Our current reference leak checker uses finalizers to verify whether an object
has reached zero references before it is garbage collected. There are multiple
problems with this mechanism, so a rewrite is in order.

With finalizers, there is no way to guarantee that a finalizer will run before
the program exits. When an unreachable object with a finalizer is garbage
collected, its finalizer will be added to a queue and run asynchronously. The
best we can do is run garbage collection upon sandbox exit to make sure that
all finalizers are enqueued.

Furthermore, if there is a chain of finalized objects, e.g. A points to B
points to C, garbage collection needs to run multiple times before all of the
finalizers are enqueued. The first GC run will register the finalizer for A but
not free it. It takes another GC run to free A, at which point B's finalizer
can be registered. As a result, we need to run GC as many times as the length
of the longest such chain to have a somewhat reliable leak checker.

Finally, a cyclical chain of structs pointing to one another will never be
garbage collected if a finalizer is set. This is a well-known issue with Go
finalizers (https://github.com/golang/go/issues/7358). Using leak checking on
filesystem objects that produce cycles will not work and even result in memory
leaks.

The new leak checker stores reference counted objects in a global map when
leak check is enabled and removes them once they are destroyed. At sandbox
exit, any remaining objects in the map are considered as leaked. This provides
a deterministic way of detecting leaks without relying on the complexities of
finalizers and garbage collection.

This approach has several benefits over the former, including:
- Always detects leaks of objects that should be destroyed very close to
  sandbox exit. The old checker very rarely detected these leaks, because it
  relied on garbage collection to be run in a short window of time.
- Panics if we forgot to enable leak check on a ref-counted object (we will try
  to remove it from the map when it is destroyed, but it will never have been
  added).
- Can store extra logging information in the map values without adding to the
  size of the ref count struct itself. With the size of just an int64, the ref
  count object remains compact, meaning frequent operations like IncRef/DecRef
  are more cache-efficient.
- Can aggregate leak results in a single report after the sandbox exits.
  Instead of having warnings littered in the log, which were
  non-deterministically triggered by garbage collection, we can print all
  warning messages at once. Note that this could also be a limitation--the
  sandbox must exit properly for leaks to be detected.

Some basic benchmarking indicates that this change does not significantly
affect performance when leak checking is enabled, which is understandable
since registering/unregistering is only done once for each filesystem object.

Updates #1486.

PiperOrigin-RevId: 338685972

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