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 in turn requires locking). 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.