MESOS-3896: Add accounting for reservation slack in the allocator

Description:

The target of this JIRA is to offer the allocation slack resources to the framework. Here're the key points of this design:

• It's different with oversubscription: oversubscription "dynamic adjust resources", make cluster has more resources (revocable); but Optimistic Offer lends reserved resources to other framework (did not increase the total resources i cluster).
• For allocation slack resources, it should NOT included in Quota.
• After Quota, remainClusterResource.contains should NOT account ALLOCATION_SLACK; for example, if 40 CPU in remainClusterResources, in one slave, 30 CPU is "unreserved + reserved" and 20 CPU is ALLOCATION_SLACK, master should offer those resources to the framework.
• In DRF, should ALLOCATION_SLACK account into the sorter?
• Currently, allocator assign all ALLOCATION_SLACK resources to the first framework; it should also be re-visit in MESOS-3765 (Make offer size adjustable (granularity)).
• I'm also thinkging to add a sorter for allocation slack after DRF; but it seems overkilled.

JIRAs of Design

MESOS-XXX: Optimsistic accounter

class HierarchicalAllocatorProcess 
{
  struct Slave
  {
    ...
    struct Optimistic 
    {
      Resources total; // The total allocation slack resources
      Resources allocated; // The allocated allocation slack resources
    };

    Optimistic optimistic;
  };
}

MESOS-4146: flatten & allocationSlack for Optimistic Offer

class Resources
{
    // Returns a Resources object with the same amount of each resource
    // type as these Resources, but with all Resource objects marked as
    // the specified `RevocableInfo::Type`; the other attribute is not
    // affected.
    Resources flatten(Resource::RevocableInfo::Type type);

    // Return a Resources object that:
    //   - if role is given, the resources did not include role's reserved
    //     resources.
    //   - the resources's revocable type is `ALLOCATION_SLACK`
    //   - the role of resources is set to "*"
    Resources allocationSlack(Option<string> role = None());
}

MESOS-XXX: Allocate the allocation_slack resources to framework

void HierarchicalAllocatorProcess::allocate(
    const hashset<SlaveID>& slaveIds_)
{
  foreach slave; foreach role; foreach framework
  {
    Resource optimistic;

    if (framework.revocable) {
      Resources total = slaves[slaveId].optimistic.total.allocationSlack(role);
      optimistic = total - slaves[slaveId].optimistic.allocated;
    }

    ...
    offerable[frameworkId][slaveId] += resources + optimistic;

    ...
    slaves[slaveId].optimistic.allocated += optimistic;
  }
}

Here's some consideration about ALLOCATION_SLACK:

1. 'Old' resources (available/total) did not include ALLOCATION_SLACK
2. After Quota, remainingClusterResources.contains should not check ALLOCATION_SLACK; if there no enough resources, master can still offer ALLOCATION_SALCK resources.
3. In sorter, it'll not include ALLOCATION_SLACK; as those resources are borrowed from other role/framework
4. If either normal resources or ALLOCATION_SLACK resources are allocable/!filtered, it can be offered to framework
5. Currently, allocator will assign all ALLOCATION_SALCK resources in slave to one framework

MESOS-XXX: Update ALLOCATION_SLACK for dynamic reservation (updateAllocation)

void HierarchicalAllocatorProcess::updateAllocation(
    const FrameworkID& frameworkId,
    const SlaveID& slaveId,
    const vector<Offer::Operation>& operations)
{
    ...
    Try<Resources> updatedOptimistic =
        slaves[slaveId].optimistic.total.apply(operations);
    CHECK_SOME(updatedTotal);

    slaves[slaveId].optimistic.total =
        updatedOptimistic.get().stateless().reserved().flatten(ALLOCATION_SLACK);
    ...
}

MESOS-XXX: Add ALLOCATION_SLACK when slaver register/re-register (addSlave)

void HierarchicalAllocatorProcess::addSlave(
    const SlaveID& slaveId,
    const SlaveInfo& slaveInfo,
    const Option<Unavailability>& unavailability,
    const Resources& total,
    const hashmap<FrameworkID, Resources>& used)
{
  ...
  slaves[slaveId].optimistic.total =
      total.stateless().reserved().flatten(ALLOCATION_SLACK);
  ...
}

No need to handle removeSlave, it'll all related info from slaves including optimistic.

MESOS-XXX: return resources to allocator (recoverResources)

void HierarchicalAllocatorProcess::recoverResources(
    const FrameworkID& frameworkId,
    const SlaveID& slaveId,
    const Resources& resources,
    const Option<Filters>& filters)
{
  if (slaves.contains(slaveId))
  {
    ...
    slaves[slaveId].optimistic.allocated -= resources.allocationSlack();
    ...
  }
}