Rework starting/stopping executors

There was at least one error, likely a race condition, with the
previous code which could cause more than one ExecutorWorkerThread for
a node to be spawned.  In particular, I think the bogus comparison in
ComputerListenerImpl may have a large part in that (it checked to see
if a _Computer_ object was in a list of _Thread_ objects).

To improve reliability around adding and removing nodes, all related
functionality is moved to the GearmanProxy class.  Any methods (most
of them) that have to do with starting or stopping worker threads are
synchronized on the GearmanProxy monitor (the important parts of most
threads were already synchronized on the worker list before, so this
should not be much of a performance change).

The methods that start management and executor threads now do their
own checking to verify that such threads do not already exist, making
it so that calling them is more idempotent.  Existing checks external
to the class have been removed (these were likely somewhat racy).

To avoid keeping redundant data structures, the node availability list
is removed, and instead if we need to find an availability object, we
walk the list of worker threads and compare to their nodes.  Because
we do this so much, the list of worker and management threads are
changed to use those explicit classes instead of
AbstractWorkerThreads.

The accessor methods for the internal lists of worker threads is
removed to ensure that they are only managed through GearmanProxy.
This changed some unit tests and required the removal of one complete
test (which was not doing much more than verifying the addition
operator).

Also, when stopping ExecutorWorkerThreads, stop all of the ones
associated with a node.

When a computer goes offline, Computer.getNode() returns null, so we
can't know which workers should be deleted.  Instead of using Nodes as
keys for our workers, use Computers instead and change everything to
use Computer (most functions needed Computer rather than Node anyway),
and in the few remaining places where a Node is needed, convert the
other direction.

Change-Id: Ia5084579317f972400069cc3e84db4e0b6560a80