adjuncts are like static resources, and they should be available without read access, or else page rendering can fail

- md is not concurrency safe, so the whole thing needs to be synchronized or we need a pool, like ThreadLocal
- There's a convenience method that creates hex string

Submitted by Marco Rothe

- Pass TaskListener to enable listeners to report what they are doing
- Added another callback to report a failure

These changes allow for a Node to check configuration values and
potentially veto bringing up a Node that is otherwise in demand.  For
example the Lab Manager plugin could be extended to allow for only 5
slaves in a given cloud to be online at given time.

Signed-off-by: Tom Rini <tom_rini@mentor.com>

- AbstractProject.getBuildingUpstream/getBuildingDownstream()
  should be symmetric.
- It's unlikely for us to add another category of items,
  so reverted getUnblockedItems() implementation to the older one
  that was more efficient.
- ItemList was meant to be internal, so keep it as such,
  at least for now.

Changed logic to build list of unblocked items in the queue safely so it will not break if a new category of queue items is added

[FIXED JENKINS-8929] Changed the definition of a building downstream project to one that is Building, Waiting, Pending or Buildable
but NOT blocked.  This prevents the deadlock condition where upstream project A is blocked on downstream project B, which is
blocked on upstream project A - now in this scenario B will remain blocked but A will build, eventually freeing up B.