use extend to implement heartbeat

[seamusabshere]

(this PR depends on #20)

here's a tested example of implementing a heartbeat using extend

lock_manager.lock(resource, ttl) do
  # this takes out a lock every second until the ttl is used up (plus a remainder at the end)
  # if this process is killed, the lock will expire quickly
end

it looks complicated, but that's a good reason that the library should implement it (i think?)

cc @antirez @maltoe @leandromoreira

[maltoe]

@seamusabshere Sorry for not commenting on this so far. I think you're right about this should be part of some library, but not of the core locking one, and could probably be even totally agnostic of the underlying lock technology. At the very least it adds far too much complexity to Redlock::Client.