Removed obsolete setting of framework id in executor.

Split from https://reviews.apache.org/r/19795/. This patch removes an
obsolete piece of code in the slave which sets the framework id in the
executor info. It has already been marked for deletion and significantly
simplifies the new reliable reregistration code.

Review: https://reviews.apache.org/r/19877
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
Improved Javadoc (use @param, @return, etc).

Review: https://reviews.apache.org/r/19859