Feature/fix race condition in resource manager

Resource manager maintains a hash table of resource repositories.
Most accesses to this hash table are synchronized, however a few
are not. This lead to the following exception:

    org.ops4j.pax.exam.TestContainerException: [startedResourceModelHasNameTest(net.i2cat.mantychore.repository.tests.MantychoreRepositoryIntegrationTest): null]
	at org.ops4j.pax.exam.invoker.junit.internal.JUnitProbeInvoker.invokeViaJUnit(JUnitProbeInvoker.java:112)
	at org.ops4j.pax.exam.invoker.junit.internal.JUnitProbeInvoker.findAndInvoke(JUnitProbeInvoker.java:89)
	at org.ops4j.pax.exam.invoker.junit.internal.JUnitProbeInvoker.call(JUnitProbeInvoker.java:72)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.ops4j.pax.exam.rbc.internal.RemoteBundleContextImpl.remoteCall(RemoteBundleContextImpl.java:86)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:303)
	at sun.rmi.transport.Transport$1.run(Transport.java:159)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
	at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
	at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)
    Caused by: java.util.ConcurrentModificationException
	at java.util.Hashtable$Enumerator.next(Hashtable.java:1031)
	at org.opennaas.core.resources.ResourceManager.listResources(ResourceManager.java:115)
	at net.i2cat.mantychore.repository.tests.MantychoreRepositoryIntegrationTest.startedResourceModelHasNameTest(MantychoreRepositoryIntegrationTest.java:226)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:30)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
	at org.ops4j.pax.exam.invoker.junit.internal.ContainerTestRunner.runChild(ContainerTestRunner.java:58)
	at org.ops4j.pax.exam.invoker.junit.internal.ContainerTestRunner.runChild(ContainerTestRunner.java:32)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:136)
	at org.ops4j.pax.exam.invoker.junit.internal.JUnitProbeInvoker.invokeViaJUnit(JUnitProbeInvoker.java:108)
	... 21 more

Although Hashtable itself is synchronized, an iterator over
Hashtable may not be interleaved with modifications to the
hash table. The solution is thus to synchronize all access
to the hash table.

The patch fixes the problem and also cleans up the code a bit:

- Uses HashMap rather than the legacy Hashtable class.

- Adds @Override annotations.

- Removes some unnecessary collection allocations.

- Simplifies a number of loops over collections.

- Prevents exposing internal data structures.

- Avoid breaking encapsulating caused by casting interfaces
  to implementations.

Testing done: Unit and integration tests pass.