Collection was modified; enumeration operation may not execute

[grokys]

When running tests, after #41 I've started seeing the following exception:

System.InvalidOperationException
  HResult=0x80131509
  Message=Collection was modified; enumeration operation may not execute.
  Source=System.Private.CoreLib
  StackTrace:
   at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion()
   at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at FlaUI.WebDriver.Session.EvictUnavailableElements() in D:\projects\FlaUI\FlaUI.WebDriver\src\FlaUI.WebDriver\Session.cs:line 133
   at FlaUI.WebDriver.SessionCleanupService.EvictUnavailableElements(ISessionRepository sessionRepository) in D:\projects\FlaUI\FlaUI.WebDriver\src\FlaUI.WebDriver\SessionCleanupService.cs:line 47
   at FlaUI.WebDriver.SessionCleanupService.DoWork(Object state) in D:\projects\FlaUI\FlaUI.WebDriver\src\FlaUI.WebDriver\SessionCleanupService.cs:line 39
   at System.Threading.TimerQueueTimer.<>c.<.cctor>b__27_0(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)

Looks like firing the cleanup on a timer is causing a problem whereby a query is running at the same time, which is adding an item to the dictionary.