[MINOR] Implement persistent id for proxies and servers

[MINOR] Implement persistent id for proxies and servers

This patch adds a possibility to set a persistent id for a proxy/server.
Now, even if some proxies/servers are inserted/deleted/moved, iids and
sids can be still used reliable.

Some people add servers with tricky names (BACKEND or FRONTEND for example).
So I also added one more field ('type') to distinguish between a
backend (0), frontend (1) and server (2) without complicated logic:
if name==BACKEND and sid==0 then type is BACKEND else type is SERVER,
etc for a FRONTEND. It also makes possible to have one frontend with more
than one IP (a patch coming soon) with independed stats - for example to
differs between remote and local traffic.

Finally, I added documentation about the CSV format.

This patch depends on '[MEDIUM] Implement "track [<backend>/]<server>"'