Remove "strict registry" code

Once PARTITION_AWARE frameworks are supported, we should eventually remove the code that supports the "non-strict" semantics in the master. That is:

1. The master will be "strict" in Mesos 1.1, in the sense that master behavior will always reflect the content of the registry and will not change depending on whether the master has failed over. The exception here is that for non-PARTITION_AWARE frameworks, we will only kill such tasks on a reregistering agent if the master hasn't failed over in the meantime. i.e., we'll remain backwards compatible with the previous "non-strict" semantics that old frameworks might depend on.
2. The "strict" semantics will be less problematic, because the master will no longer be killing tasks and shutting down agents.