Added communication between Serve and CLI using KV table

[juanfont]

The in-memory clientsPolling map is only available from the Headscale process serving the API. This means that some changes performed by the CLI (which is a different process) are not propagated immediately to the polling clients.

This is what is happening in #52. Nodes are successfully removed from the DB, but the connected machines do not receive an updated node Map until their client decides to close the long poll and reconnect.

I have added a new worker that polls the KV table we had since the beginning in Headscale DB schema. Namespaces whose nodes should be sent an updated map are added to the namespaces_pending_updates key.

The hardcoded polling time is 5000 msecs.

[juanfont]

I am not 100% happy with this, as it looks a bit convoluted. But it works(tm).