Implement room purge after last local user leaves. (resolves #83)

Consume a state_lock for room delete call.

Signed-off-by: Jason Volk <jason@zemos.net>

tuwunel-example.toml

@@ -1657,6 +1657,17 @@
 #
 #hydra_backports = false
 
+# Delete rooms when the last user from this server leaves. This feature is
+# experimental and for the purpose of least-surprise is not enabled by
+# default but can be enabled for deployments interested in conserving
+# space. It may eventually default to true in a future release.
+#
+# Note that not all pathways which can remove the last local user
+# currently invoke this operation, so in some cases you may find the room
+# still exists.
+#
+#delete_rooms_after_leave = false
+
 #[global.tls]
 
 # Path to a valid TLS certificate file.