Fixed a regression in ExecutorInfoChecker to allow an executor with the same id but different executor info as long as it is on a different slave.

Review: https://reviews.apache.org/r/23187
Added the low level scheduler example using libprocess.

Review: https://reviews.apache.org/r/22539
Fixed flaky OrphanTasks test.

Added Clock::settle after StartMaster() to ensure the master finishes executing _recover().

Review: https://reviews.apache.org/r/23001
Made sure Cluster::Masters::start() returns when the Master is ready for incoming requests. i.e., after it has finished executing _recover().

Review: https://reviews.apache.org/r/22857