Fix JSONRPC server shutdown race

[bneradt]

JSONRPC server shutdown can race with worker thread startup. When the
worker starts after stop_thread(), it restores the running flag and
polls a closed socket indefinitely. This causes test_jsonrpcserver and
process shutdown to hang.

This marks the socket server as running before the worker is created,
so a concurrent stop cannot be overwritten. It also passes the owning
server to the worker instead of relying on the mutable global server
pointer.

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Pull request overview

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src/mgmt/rpc/server/RPCServer.cc:67

• The worker thread runs the optional cb_init (e.g. TSThreadInit) and stores the returned handle in _rpcThread, but the matching cb_destroy is never invoked. For TSThreadDestroy specifically, the API requires the same thread that created the TSThread to destroy it (see src/api/InkIOCoreAPI.cc:202-205), so destroying it from stop_thread (caller thread) would be incorrect. This should be cleaned up in the worker thread after _socketImpl->run() returns to avoid leaking per-thread state and to respect the required destroy-thread affinity.

  if (server->_init) {
    server->_rpcThread = server->_init();
  }
  server->_socketImpl->run();
  Dbg(dbg_ctl, "Socket stopped");

[cmcfarlen]

Cherry-picked to the 10.2.x branch as 1427ebf for the 10.2.0 release.