Adds optional transcoding parameters to a Mount. Each node will then
start a master election for every transcoded stream, and the master will
run liquidsoap to re-encode the data. Transcoding mounts are identical
in other respects to normal mounts, except that liquidsoap acts as a
source for them.

Refactor the load balancing code using a generic traffic control
abstraction that gives the user more control over the final logic (for
the moment this is configured via a command-line parameter).

Adds a new metric to NodeStatus (MaxListeners) so that it is available
to the traffic control logic.

This change allows us to introduce changes to the inner workings of the
cluster that break backwards compatibility, allowing a smooth transition
to the new format.

Increase robustness of the config watcher by reloading the full
configuration on Etcd errors (fixes an issue with the very common error
401).

Make sure that the Icecast config reload is only triggered if the
configuration has actually changed.

Added a way to stop the node properly, which ensures that we are
tracking all the resources and that there are no deadlocks in the
internal communications.

MasterElection now closes the update channel when it's done.

Master election code should be more robust now, this commit eliminates
the discrepancy between role and master info (previously it was possible
for these two to be non synchronized).

This commit also includes an in-memory etcd mock, which implements
enough of the etcd interface to test the masterelection code.