JSON Contract resolver needs to be reused

[ProfFrugal]

JSON Contract resolver is being allocated over and over again, causing more allocation and CPU usage due to reflection and jitting.

For more information: check:

https://www.linkedin.com/feed/update/urn:li:activity:7096144517917016064/
https://frugalcafe.beehiiv.com/p/fc17-real-world-example-not-reusing-json-contract-resolver

[AArnott]

Thanks for sharing this. I'm not sure it's as simple as it may look though. We modify the contracts that this DefaultContractResolver object returns in a way that varies based on the instance of the MarshalContractResolver (and in turn, based on the instance of the JsonMessageFormatter). If we were to share a common DefaultContractResolver, the cached contracts would be shared across these instances, and the mutation may corrupt contracts that were previously returned. Such a bug would likely only manifest when multiple threads are actively serializing/deserializing at once, so simple functional tests may not catch it.