For JSON-RPC 2.0 requests we need to distinguish between
a missing "id" field and "id":null. This is accomplished
by making the JSONRPCRequest id property a
std::optional<UniValue> with a default value of
UniValue::VNULL.
A side-effect of this change for non-2.0 requests is that request which do not
specify an "id" field will no longer return "id": null in the response.
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