recovery: improve monitor thread

Use counters and timers from Stats class and built-in python HTTPServer.
Now monitor provides HTTP interface which admins can esily use via curl/wget.
Also save stat.txt file for future post-mortems / local monitoring.

PS. JSON interface will come soon.
recovery: moved lock to the very start of recovery
recovery: improve logging

Now we saving all output to rotating logfile, so admins can use them in
post-mortems.
recovery: use dnet_recovery prefix in /var/tmp

This should minimize chances of collision with other software
recovery: use mkdir -p for creating temp dir