Moved slave disconnection into new Master::disconnect(Slave*).

Moved code to disconnect a checkpointing slave out of Master::exited()
and into a new disconnect(Slave), to be re-used by slave
authentication code. Also renamed deactivateSlave() to
shutdownSlave(), since it actually sends a ShutdownMessage and calls
removeSlave(). This avoids confusion with deactivateFramework, which
is more analagous to disconnect(Slave).

Review: https://reviews.apache.org/r/19006