workaround db connection getting closed at runtime...

[amirouche]

... for unknown reasons, the db connection is closed at runtime, due
to a high load. Here is an example traceback:

 [2021-08-15 17:00:46,840] ERROR in app: Exception on /a/[/r/EtymologicallyRelatedTo/,/c/en/drag/,/c/nds/dragge/] [GET]
 Traceback (most recent call last):
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
     response = self.full_dispatch_request()
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
     rv = self.handle_user_exception(e)
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
     reraise(exc_type, exc_value, tb)
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
     raise value
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
     rv = self.dispatch_request()
   File "/home/conceptnet/env/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
     return self.view_functions[rule.endpoint](**req.view_args)
   File "/home/conceptnet/conceptnet5/web/conceptnet_web/web.py", line 133, in browse_node
     results = responses.lookup_paginated(path, offset=offset, limit=limit)
   File "/home/conceptnet/conceptnet5/conceptnet5/api.py", line 150, in lookup_paginated
     found = FINDER.lookup(term, limit=(limit + 1), offset=offset)
   File "/home/conceptnet/conceptnet5/conceptnet5/db/query.py", line 142, in lookup
     return self.lookup_assertion(uri)
   File "/home/conceptnet/conceptnet5/conceptnet5/db/query.py", line 194, in lookup_assertion
     cursor = self.connection.cursor()
 psycopg2.InterfaceError: connection already closed

This patch will re-create the connection any time the connection is
closed. Bonus: factor connection creation inside a property called
connection.

ref: #306