Allocation in batch instead of execute it every-time when addSlave/addFramework.

Currently, allocate() are executed every-time when a new slave/framework are registered; if there're lots of agent start all most the same time, the allocation will keep running for a while. It's acceptable behaviour to allocate resources in next allocation cycle. But when a task is finished, it's better to allocate ASAP although there's performances issues; refer to MESOS-3078 for more detail on short running tasks.