In order to be able to make progress on the sharded transactions work before the SessionCatalog is made decorable and made available on mongos, we should just create a simple std::map<LogicalSessionId, TransactionRuntimeState> in order to be able to store per-session transaction information. In order to abstract this temporary map, the API for TransactionRuntimeState should behave like a decoration on OperationContext, even though it might not be one at first.

E.g.: static TransactionRuntimeState* TransactionRuntimeState::get(OperationContext* opCtx);

Should include some basic unit-tests.