HistoryAppender no longer uses ProcessHistoryRegistry

To allow for thread-safe uses of the ProcessHistoryRegistry, the HistoryAppender no longer updates the ProcessHistoryRegistry to hold its appended history. Now it just returns a shared_ptr to the new or cached ProcessHistory. The shared_ptr is now owned by all Principals which are currently processing with that history.
To simplify the sharing, we only cache the last ProcessHistory and no longer hold a map to all previously seen histories. This will allow us to have one HistoryAppender per Stream when used by the SubProcess.
Updated to match new HistoryAppender API

The HistoryAppender now returns a shared_ptr<ProcessHistory const> so processHistoryPtr_ was changed to match. In addition, the fillPrincipal function no longer needs to modify the ProcessHistoryRegistry so the argument is now const.
fill*Principal method no longer modifies the ProcessHistoryRegistry.

The underlying Principal::fillPrincipal no longer modifies the ProcessHistoryRegistry and therefore takes a const reference. The fill*Principal methods were updated to match. This makes it clearer when reasoning about thread safety.
RunPrincipal::fillRunPrincipal no longer modifies the ProcessHistoryRegistry and RunPrincipal now holds the reducedProcessHistoryID

-Principal::fillPrincipal no longer modifies the ProcessHistoryRegistry so now takes a const argument. fillRunPrincipal was updated to match.
-The RunPrincipal now holds onto the reducedProcessHistoryID which is used to uniquely identify a Run. This makes identification of a Run easier for the SubProcess when it is time to write that Run.
SubProcess has own copy of ProcessHistoryRegistry

Now has its own copy of the ProcessHistoryRegistry as preparation for thread-safety. As part of the change, now get the parent’s reducedProcessHistoryID from the RunPrincipal and not from the parent ProcessHistoryRegistry (which is no longer accessible).
Full thread-safety will require each Stream to have its own ProcessHistoryRegistry and HistoryAppender.
EventPrcocessor uses new SubProcess constructor and reducedProcessHistory from RunPrincipal

The SubProcess no longer takes a ProcessHistoryRegistry as an argument to its constructor since it now has its own internal copy. This avoids possible updates of a ProcessHistoryRegistry from different threads.
In addition, RunPrincipal now holds its reducedProcessHistoryID so the EventProcessor now uses it from the RunPrincipal and not the Source.
Pass const ProcessHistoryRegistry to fill*Principal

The various fill*Principal member functions were changed to take a const ProcessHistoryRegistry. This change just swaps out using processHistoryRegistryForUpdate() and now use processHistoryRegistry() to make it clear that no change is required.
SubProcess has separate history info for each Run/Lumi/Stream

If the SubProcess had only one ProcessHistoryRegistry, that registry could be updating at the same time the HistoryAppender was reading from it. Therefore we need separate ProcessHistoryRegistry and HistoryAppender for each concurrent Run, Lumi and Stream.
Merge branch 'CMSSW_7_0_X' into makeProcessHistoryRegistryThreadSafe

Attempt to do a rebase failed without an adequate explanation from git as to why it failed.
Merge pull request #927 from Dr15Jones/makeProcessHistoryRegistryThreadSafe

Make use of ProcessHistoryRegistry thread safe