watcher: use a PriorityQueue

Commit 839d24e61664b91a06ef972102111a9c550d6eb3 was meant to use a
priority queue. This is not the case, switch to priority queue. This
allows connection-related events to be handled earlier and discards a
few backtraces. This does not change the overall logic of the FSM.
zkfarmer: default log level to INFO for join/export

By default, the logging level is WARN which is too low for a
daemon. When using `join` or `export` commands, we default to
INFO. Also, `-v` will increase the log level while `-q` will set
logging level to WARN even for `join` and `export`.
Merge pull request #21 from vincentbernat/fix/default-logging

zkfarmer: default log level to INFO for join/export
Merge pull request #20 from vincentbernat/fix/priorityqueue

watcher: use a PriorityQueue