Fix issue with closing sockets in plugin groups

As per Boost.Asio's manual, an explicit `socket.shutdown()` is needed
before calling `close()`. For some reason this worked fine in almost
every situation, but when hosting both a plugin hosted within a group
host process and a normal individually hosted plugin within a single
process, and then removing those two plugins in order, the
`host_vst_dispatch` socket of the first plugin never got closed. This
would hang the entire shutdown sequence to hang on the
`dispatch_handler` jthread.

First discovered in #45

src/plugin/host-process.cpp

@@ -262,5 +262,7 @@ void GroupHost::terminate() {
     // There's no need to manually terminate group host processes as they will
     // shut down automatically after all plugins have exited. Manually closing
     // the dispatch socket will cause the associated plugin to exit.
+    host_vst_dispatch.shutdown(
+        boost::asio::local::stream_protocol::socket::shutdown_both);
     host_vst_dispatch.close();
 }