Sequence all operations on the agent

The resolution of MESOS-8582 requires that an asynchronous step be added to the code path which applies speculative operations like RESERVE and CREATE on the agent. In order to ensure that the FrameworkInfo associated with an incoming operation will be successfully retained, we must first unschedule GC on the framework meta directory if the framework struct does not exist but that directory does. By introducing this asynchronous step, we allow the possibility that an operation may be executed out-of-order with respect to an incoming dependent LAUNCH or LAUNCH_GROUP.

For example, if a scheduler issues an ACCEPT call containing both a RESERVE operation as well as a LAUNCH operation containing a task which consumes the new reserved resources, it's possible that this task will be launched on the agent before the reserved resources exist.

While we already sequence task launches on a per-executor basis, the aforementioned corner case requires that we sequence all offer operations on a per-framework basis.