[FIXED JENKINS-21044] - Throttling blocks the Jenkins queue

Seems the issue was in improper usage of WeakHashMap (see analysis from @centic).
I've managed to reproduce the behavior in the following case:
* There is a big number of jobs/configurations with throttling
* The builds queue is not empty
// Seems that ThrottleQueueTaskDispatcher tries to access the data before the complete loading of the plugin's configuration.

This fix provides an explicit locking of any load operations + manual cleanup of erroneous cache data, which goes to persistence in 1.8.1
Resolves https://issues.jenkins-ci.org/browse/JENKINS-21044

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
[JENKINS-21044] - Don't create a new HashMap after the load operation

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge branch 'JENKINS_21044_fix' of github.com:synopsys-arc-oss/throttle-concurrent-builds-plugin

Conflicts:
	src/main/java/hudson/plugins/throttleconcurrents/ThrottleJobProperty.java