remove context.inbox in favour of a context.inbox_thread following the mvbox_thread and sentbox_thread patterns. Also some streamlining of shutdown logic.

src/config.rs

@@ -121,7 +121,7 @@ impl Context {
             }
             Config::InboxWatch => {
                 let ret = self.sql.set_raw_config(self, key, value);
-                interrupt_imap_idle(self);
+                interrupt_inbox_idle(self, true);
                 ret
             }
             Config::SentboxWatch => {