[System.ServiceModel] Fix "collection was modified" exception in tests

[System.ServiceModel] Fix "collection was modified" exception in tests

We sometimes see the following exception in the ServiceModel tests:

```
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
  at System.ThrowHelper.ThrowInvalidOperationException (ExceptionResource resource) [0x00000] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/external/referencesource/mscorlib/system/throwhelper.cs:99
  at System.Collections.Generic.List`1+Enumerator[T].MoveNextRare () [0x00016] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/external/referencesource/mscorlib/system/collections/generic/list.cs:1180
  at System.Collections.Generic.List`1+Enumerator[T].MoveNext () [0x00050] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/external/referencesource/mscorlib/system/collections/generic/list.cs:1174
  at System.Linq.Enumerable.FirstOrDefault[TSource] (IEnumerable`1 source, System.Func`2 predicate) [0x00041] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/external/referencesource/System.Core/System/Linq/Enumerable.cs:960
  at System.ServiceModel.Channels.Http.HttpListenerManager.TryDequeueRequest (System.ServiceModel.Dispatcher.ChannelDispatcher channel, TimeSpan timeout, System.ServiceModel.Channels.Http.HttpContextInfo& context) [0x00016] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpListenerManager.cs:93
  at System.ServiceModel.Channels.Http.HttpListenerManager.TryDequeueRequest (System.ServiceModel.Dispatcher.ChannelDispatcher channel, TimeSpan timeout, System.ServiceModel.Channels.Http.HttpContextInfo& context) [0x000ab] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpListenerManager.cs:104
  at System.ServiceModel.Channels.Http.HttpReplyChannel.TryReceiveRequest (TimeSpan timeout, System.ServiceModel.Channels.RequestContext& context) [0x00003] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/mcs/class/System.ServiceModel/System.ServiceModel.Channels.Http/HttpReplyChannel.cs:154
  at System.ServiceModel.Channels.ReplyChannelBase+<BeginTryReceiveRequest>c__AnonStorey0.<>m__0 (TimeSpan tout, System.ServiceModel.Channels.RequestContext& ctx) [0x00056] in /var/lib/jenkins/workspace/test-mono-mainline/label/debian-amd64/mcs/class/System.ServiceModel/System.ServiceModel.Channels/ReplyChannelBase.cs:129
```

I managed to repro this consistently on the debian-7-x64-1 Jenkins machine by running the test in a loop with the following code for about 15mins:
    while [ $? -eq 0 ]; do make check FIXTURE=System.ServiceModel.Dispatcher; done

The problem happens when RegisterListenerCommon/UnregisterListenerCommon is invoked through HttpChannelListener.OnOpen/OnAbort/OnClose which modifies the 'Entries' collection on another thread.

The fix is to add locking around the collection accesses.