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