Use object_id as cache key in CachingResolver

[dmilisic]

What does this PR do?
Fix #5718 Segmentation fault in CachingResolver

Motivation:
Ocasional segfaults after upgrading from ddtrace (v1) to datadog (v2)

Change log entry

Additional Notes:
When using rails with multiple threads, each thread will have its own connection configuration Hash with a different object_id. So we can expect more cache entries, but we eliminate the overhead of comparing hashes.

We're running this fix in production for ~2 weeks without a single segfault. Previously, segfaults ocurred on a daily basis.

How to test the change?

Segmentation faults are difficult to reproduce. There should be no change in behaviour.

[ivoanjo]

Thanks for sharing this! If the workaround I mentioned in #5718 (comment) is not enough, this provides us a really good hint where the interaction of the code in the Ruby VM might be going wrong... 👀