disconnected clients: ensure servers meet minimum required version

[DerekStrickland]

Note for Reviewer: This is a pragmatic compromise that might need discussion.

• The CoreScheduler has a direct reference to the nomad.Server and thus its members and is responsible for
  performing server version checks for core jobs.
• Initially, I tried exposing this functionality via the Scheduler interface, however, the GenericScheduler cannot have a
  direct reference to the nomad.Server because it is in the scheduler package and that creates an import cycle.
• The Worker fulfills the Planner interface, is made available to the GenericScheduler by that interface, and also
  has a reference to the nomad.Server. So the Planner interface was extended instead.

Overview

This PR contains the logic necessary to ensure clusters running mixed version servers do not enable the max_client_disconnect feature until all servers meet the minimum required version.

• bf71504
  • Exposes the ServersMeetMinimumVersion utility function via the Planner interface.
• 6e60a2c
  • Modifies filterByTained to include a boolean flag indicating whether the caller supports disconnected clients.
  • If not, the disconnected and reconnecting allocSets are never populated.
  • Effectively short-circuits disconnect/reconnect logic from ever triggering in the reconciler.
• b41401b
  • Adds the version check logic in the GenericScheduler
  • Adds a field to the allocReconciler.
  • ModifiesNewAllocReconciler to accept a parameter to set the field.
  • Plumbs it through to all calls tofilterByTainted.
• cfc5f49
  • Updates affected/dependent tests

[tgross]

LGTM once my comment about the tests is addressed.

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