close nostrdb and disconnect from relays on logout

This was causing crashing and corruption issues. This should have been
handled by the garbage collector, but for some reason old references
still hang around.

Add a "close" method to DamusState which disconnects from relays and
closes nostrdb.

Changelog-Fixed: Fix crash when logging out and switching accounts
Changelog-Fixed: Fix persistent local notifications even after logout
Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/Ndb.swift

@@ -176,6 +176,7 @@ class Ndb {
     }
     
     func close() {
+        guard !self.closed else { return }
         self.closed = true
         ndb_destroy(self.ndb.ndb)
     }