Use event-local memory pool when reading RawEvent
This uses the tools of !2270 (merged) when reading raw events from MDF.
There is a slight complication thanks to a bug in ROOT: https://sft.its.cern.ch/jira/browse/ROOT-10526 A fix has been proposed, but I haven't been able to test it yet. The fix should make it to the 6.20 release, and presumably will be available via the LCG dev3 and/or dev4 slots before then. The effect is that ROOT I/O of LHCb::RawEvent is not possible with this MR and without the ROOT fix.
I added a unit test to this MR that exposes the problem (it segfaults/fails).