rework albatross-stats

[hannesm]

There are some issues with albatross-stats:

• it's an optional dependency of albatross-daemon (which expects the stats socket to be present or not)
• there's quite some communication overhead between albatross-stats and albatross-influx
• if albatross-stats goes away, all you can do is restart albatross-daemon

A solution from this mornings brainstorming:

• revise the protocol, so an albatross-stats can connect to the albatross-daemon socket, and gets (a) running unikernels at connect-time (b) whenever a unikernel starts or stops, the albatross-stats connection is notified
• merge albatross-influx into albatross-stats and avoid any re-marshalling of data. when there's at some point interest in prometheus or other wire formats, we can cope (but still only need a single stats collecting and reporting daemon)

This would as well solve #55 by just not encoding and decoding this information ;)

[hannesm]

The protocol has been inverted (the stats daemon now connects to the albatross daemon) in #131.

What is left to do is the question: should stats and influx being merged? As outlined in #55 this would allow less marshalling work, though that feels like premature optimization while loosing some functionality.

[hannesm]

Since there's premature optimization mentioned in the comment above from a year back, closing this issue. When this becomes the trouble point, we can always open it.