remove restriction on slave to run single job at a time

The gearman plugin was restricting slaves to runnning
only one job at a time even though the slave may have the
ability to run multiple jobs in parallel (by having
multiple executors).  This commit fixes the restriction.

Change-Id: I62b4ff9d12474a5885c549f0b532366cb60fcbca
Don't wait for the worker thread to join

If a node is deleted, it can call the AbstractWorkerThread.stop
method while holding the monitor lock on the Jenkins object.  This
will interrupt the GearmanWorkerImpl thread, and then wait for it
to join.  However, if at that moment, the GWI is trying to set
the node offline (due to OFFLINE_NODE_WHEN_COMPLETE being set),
then the system will deadlock while that thread tries to grab
the Jenkins object monitor in order to save the new node
configuration.

Instead, just don't wait for the thread to join, which will
eventually release the monitor lock and allow the GWI thread
to complete.

Change-Id: Ic8dbdf73551feb684ac4f3e46d2ee3e5aef976db