test mjsonrpc jrpc() for thread safety

mjsonrpc jrpc() is maybe unsafe for multithread use, this happens in mhttpd if multiple rpc calls are made to the same rpc target - in mhttpd each request is handled by a different thread. on branch feature/midas-2020-12 rpc layer was reworked for thread safety, but mhttpd and jrpc() still need to be tested. K.O.