Refill cache if event not there

[hmpf]

For #135

This makes sorting very slow since it refetches ALL events.

[hmpf]

Thinking...

On restart of howitz server.

Check if cache is empty:

• If yes, get all events
• If no: sync cache with reality

Sync with reality:

• get list of all event ids
• remove removed
• loop over events existing in cache to update any changes
• add new

This is probably what should be done on loss of connection to zino server as well. The terrain might have changed.

[hmpf]

The cache will be refilled on reconnect in the error handler itself so this is no longer needed.