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

CHANGELOG.md

@@ -14,6 +14,9 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   errors and crash yabridge.
 - Fixed a regression where certain fake dropdown menus such as those used in the
   Tokyo Dawn Records plugins would close immediately when hovering over them.
+- Fixed an issue where plugins hosted within a plugin group would not shut down
+  properly in certain situations. This would cause the VST host to hang while
+  removing such a plugin.
 
 ### yabridgectl
 

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();
 }