Implement debug utility for elastio-snap

[skypodolsky]

This commit introduces the console debug utility for the elastio-snap driver. The design is the following:

• driver sends Netlink events
• the events are read by nl_debug (debugging utility)
• events are proxied by using the UDP protocol
• parsed and printed in the console if needed

The events are sent as multicast messages, so potential packet drops are possible. All messages have sequence numbers and nl_debug detects if any packet drops took place. If yes, it specifies the amount of missed packets. Despite the drops are possible, practically, they don't occur.

The utility supports different filtering that simplifies debugging:

• bio in the sector range
• bio of the specific type (read/write)
• muting of different events by groups (COW file & bio events)
• all messages are colored depending on the type to simplify reading

The Netlink part is not compiled by default, so there is a 0% chance it can degrade the driver performance. A special flag should be set manually to include it:

sudo make NETLINK_DEBUG=y

Closes #272

[skypodolsky]

That said, why is UDP used here? Is there a scenario where we need to run the driver on one host and collect metrics on another?

We might need some proxy in the future (this utility was planned as a proxy initially), so I decided to lay some foundation for that 🙂