Superseeds: protocolbuffers#13054

The object cache is fundamentally subject to race conditions.
Objects must be created before they are registered into the cache,
so if two threads try to create the same object, we'll inevitably
end up with two instances mapping to the same underlying memory.

To entirely prevent that we'd need a lot of extra locking which
I don't think is really worth it compared to a few useless allocations.

Instead we can replace `ObjectCache_Add` by a `getset` type of operation,
the extra instance is still created, but the later threads will receive
the "canonical" instance and will be able to abandon their duplicated
instance.

Additionally, this PR moves the ObjectCache implementation in Ruby,
as it's much easier to debug there, and the performance difference
is negligible. The `ObjectCache` instance is also exposed as
`Google::Protobuf::OBJECT_CACHE` to better allow to debug
potential memory issues.

Add unit tests for ObjectCache.

- hit multiple keys
- disable on jruby