HLTEDMCreator: optimize creation of handles and their keys

Creating a Read/WriteHandle from a plain string triggers the internal creation of a corresponding Read/WriteHandleKey, which has a small cost (e.g. lookup of the CLID which requires locking; this should be fixed in a future Gaudi version though). Also it prevents passing of the EventContext.

Change the code to consistently use (and re-use) handle keys. Also rename a few variables and use const-refs where relevant.

cc @tamartin