Handle Server Dropouts using authorized tags cache

[nog3]

Related to #2

If the server doesn't respond to a heartbeat the client should:

• Set a flag to start using the local authorized tags cache.
• Load the tags cache into RAM from authorised.json

When the server responds to a heartbeat, the client should:

• Make sure the flag for using the local authorized tags cache is not set.
• Unload the tags cache from RAM, if it's there.

When the server doesn't respond to an auth request for a tag, the client should:

• Load the tags cache into RAM from authorised.json
• Check against the tags cache for the current user and auth based on that.
• Set a flag to start using the local authorized tags cache.

Ideally the enabling cache during auth won't happen, due to a default 1 minute heartbeat.