Add ReservationType and ReservationInfo for dynamic reservations.

Review: https://reviews.apache.org/r/28697
Modified Resources to account for reservation type.

Review: https://reviews.apache.org/r/28698
Add dynamic reservation information to LaunchTasksMessage.

Review: https://reviews.apache.org/r/29173
Update the C++ Scheduler API.

Review: https://reviews.apache.org/r/29175
Update the master and allocator + Added a test for acquiring dynamic reservations.

Review: https://reviews.apache.org/r/29174