Sentinel: persist its unique ID across restarts.

Previously Sentinels always changed unique ID across restarts, relying
on the server.runid field. This is not a good idea, and forced Sentinel
to rely on detection of duplicated Sentinels and a potentially dangerous
clean-up and re-add operation of the Sentinel instance that was
rebooted.

Now the ID is generated at the first start and persisted in the
configuration file, so that a given Sentinel will have its unique
ID forever (unless the configuration is manually deleted or there is a
filesystem corruption).
